The following function will return an array of the parameter names of any function passed in.

var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /([^\s,]+)/g;
function getParamNames(func) {
  var fnStr = func.toString().replace(STRIP_COMMENTS, '');
  var result = fnStr.slice(fnStr.indexOf('(')+1, fnStr.indexOf(')')).match(ARGUMENT_NAMES);
  if(result === null)
     result = [];
  return result;
}

Example usage:

getParamNames(getParamNames) // returns ['func']
getParamNames(function (a,b,c,d){}) // returns ['a','b','c','d']
getParamNames(function (a,/*b,c,*/d){}) // returns ['a','d']
getParamNames(function (){}) // returns []

Edit:

With the invent of ES6 this function can be tripped up by default parameters. Here is a quick hack which should work in most cases:

var STRIP_COMMENTS = /(\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s*=[^,\)]*(('(?:\\'|[^'\r\n])*')|("(?:\\"|[^"\r\n])*"))|(\s*=[^,\)]*))/mg;

I say most cases because there are some things that will trip it up

function (a=4*(5/3), b) {} // returns ['a']

Edit: I also note vikasde wants the parameter values in an array also. This is already provided in a local variable named arguments.

excerpt from https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions_and_function_scope/arguments:

The arguments object is not an Array. It is similar to an Array, but does not have any Array properties except length. For example, it does not have the pop method. However it can be converted to a real Array:

var args = Array.prototype.slice.call(arguments);

If Array generics are available, one can use the following instead:

var args = Array.slice(arguments);

Below is the code taken from AngularJS which uses the technique for its dependency injection mechanism.

And here is an explanation of it taken from http://docs.angularjs.org/tutorial/step_05

Angular's dependency injector provides services to your controller when the controller is being constructed. The dependency injector also takes care of creating any transitive dependencies the service may have (services often depend upon other services).

Note that the names of arguments are significant, because the injector uses these to look up the dependencies.

/**
 * @ngdoc overview
 * @name AUTO
 * @description
 *
 * Implicit module which gets automatically added to each {@link AUTO.$injector $injector}.
 */

var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;
var FN_ARG_SPLIT = /,/;
var FN_ARG = /^\s*(_?)(.+?)\1\s*$/;
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
function annotate(fn) {
  var $inject,
      fnText,
      argDecl,
      last;

  if (typeof fn == 'function') {
    if (!($inject = fn.$inject)) {
      $inject = [];
      fnText = fn.toString().replace(STRIP_COMMENTS, '');
      argDecl = fnText.match(FN_ARGS);
      forEach(argDecl[1].split(FN_ARG_SPLIT), function(arg){
        arg.replace(FN_ARG, function(all, underscore, name){
          $inject.push(name);
        });
      });
      fn.$inject = $inject;
    }
  } else if (isArray(fn)) {
    last = fn.length - 1;
    assertArgFn(fn[last], 'fn')
    $inject = fn.slice(0, last);
  } else {
    assertArgFn(fn, 'fn', true);
  }
  return $inject;
}

Here is an updated solution that attempts to address all the edge cases mentioned above in a compact way:

function $args(func) {  
    return (func + '')
      .replace(/[/][/].*$/mg,'') // strip single-line comments
      .replace(/\s+/g, '') // strip white space
      .replace(/[/][*][^/*]*[*][/]/g, '') // strip multi-line comments  
      .split('){', 1)[0].replace(/^[^(]*[(]/, '') // extract the parameters  
      .replace(/=[^,]+/g, '') // strip any ES6 defaults  
      .split(',').filter(Boolean); // split & filter [""]
}  

Abbreviated test output (full test cases are attached below):

'function (a,b,c)...' // returns ["a","b","c"]
'function ()...' // returns []
'function named(a, b, c) ...' // returns ["a","b","c"]
'function (a /* = 1 */, b /* = true */) ...' // returns ["a","b"]
'function fprintf(handle, fmt /*, ...*/) ...' // returns ["handle","fmt"]
'function( a, b = 1, c )...' // returns ["a","b","c"]
'function (a=4*(5/3), b) ...' // returns ["a","b"]
'function (a, // single-line comment xjunk) ...' // returns ["a","b"]
'function (a /* fooled you...' // returns ["a","b"]
'function (a /* function() yes */, \n /* no, */b)/* omg! */...' // returns ["a","b"]
'function ( A, b \n,c ,d \n ) \n ...' // returns ["A","b","c","d"]
'function (a,b)...' // returns ["a","b"]
'function $args(func) ...' // returns ["func"]
'null...' // returns ["null"]
'function Object() ...' // returns []

function $args(func) {  
    return (func + '')
      .replace(/[/][/].*$/mg,'') // strip single-line comments
      .replace(/\s+/g, '') // strip white space
      .replace(/[/][*][^/*]*[*][/]/g, '') // strip multi-line comments  
      .split('){', 1)[0].replace(/^[^(]*[(]/, '') // extract the parameters  
      .replace(/=[^,]+/g, '') // strip any ES6 defaults  
      .split(',').filter(Boolean); // split & filter [""]
}  

// test cases  
document.getElementById('console_info').innerHTML = (
[  
  // formatting -- typical  
  function(a,b,c){},  
  function(){},  
  function named(a, b,  c) {  
/* multiline body */  
  },  
    
  // default values -- conventional  
  function(a /* = 1 */, b /* = true */) { a = a||1; b=b||true; },  
  function fprintf(handle, fmt /*, ...*/) { },  
  
  // default values -- ES6  
  "function( a, b = 1, c ){}",  
  "function (a=4*(5/3), b) {}",  
  
  // embedded comments -- sardonic  
  function(a, // single-line comment xjunk) {}
    b //,c,d
  ) // single-line comment
  {},  
  function(a /* fooled you{*/,b){},  
  function /* are you kidding me? (){} */(a /* function() yes */,  
   /* no, */b)/* omg! */{/*}}*/},  
  
  // formatting -- sardonic  
  function  (  A,  b  
,c  ,d  
  )  
  {  
  },  
  
  // by reference  
  this.jQuery || function (a,b){return new e.fn.init(a,b,h)},
  $args,  
  
  // inadvertent non-function values  
  null,  
  Object  
].map(function(f) {
    var abbr = (f + '').replace(/\n/g, '\\n').replace(/\s+|[{]+$/g, ' ').split("{", 1)[0] + "...";
    return "    '" + abbr + "' // returns " + JSON.stringify($args(f));
  }).join("\n") + "\n"); // output for copy and paste as a markdown snippet

<pre id='console_info'></pre>

Solution that is less error prone to spaces and comments would be:

var fn = function(/* whoa) */ hi, you){};

fn.toString()
  .replace(/((\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s))/mg,'')
  .match(/^function\s*[^\(]*\(\s*([^\)]*)\)/m)[1]
  .split(/,/)

["hi", "you"]

A lot of the answers on here use regexes, this is fine but it doesn't handle new additions to the language too well (like arrow functions and classes). Also of note is that if you use any of these functions on minified code it's going to go 🔥. It will use whatever the minified name is. Angular gets around this by allowing you to pass in an ordered array of strings that matches the order of the arguments when registering them with the DI container. So on with the solution:

var esprima = require('esprima');
var _ = require('lodash');

const parseFunctionArguments = (func) => {
    // allows us to access properties that may or may not exist without throwing 
    // TypeError: Cannot set property 'x' of undefined
    const maybe = (x) => (x || {});

    // handle conversion to string and then to JSON AST
    const functionAsString = func.toString();
    const tree = esprima.parse(functionAsString);
    console.log(JSON.stringify(tree, null, 4))
    // We need to figure out where the main params are. Stupid arrow functions 👊
    const isArrowExpression = (maybe(_.first(tree.body)).type == 'ExpressionStatement');
    const params = isArrowExpression ? maybe(maybe(_.first(tree.body)).expression).params 
                                     : maybe(_.first(tree.body)).params;

    // extract out the param names from the JSON AST
    return _.map(params, 'name');
};

This handles the original parse issue and a few more function types (e.g. arrow functions). Here's an idea of what it can and can't handle as is:

// I usually use mocha as the test runner and chai as the assertion library
describe('Extracts argument names from function signature. 💪', () => {
    const test = (func) => {
        const expectation = ['it', 'parses', 'me'];
        const result = parseFunctionArguments(toBeParsed);
        result.should.equal(expectation);
    } 

    it('Parses a function declaration.', () => {
        function toBeParsed(it, parses, me){};
        test(toBeParsed);
    });

    it('Parses a functional expression.', () => {
        const toBeParsed = function(it, parses, me){};
        test(toBeParsed);
    });

    it('Parses an arrow function', () => {
        const toBeParsed = (it, parses, me) => {};
        test(toBeParsed);
    });

    // ================= cases not currently handled ========================

    // It blows up on this type of messing. TBH if you do this it deserves to 
    // fail 😋 On a tech note the params are pulled down in the function similar 
    // to how destructuring is handled by the ast.
    it('Parses complex default params', () => {
        function toBeParsed(it=4*(5/3), parses, me) {}
        test(toBeParsed);
    });

    // This passes back ['_ref'] as the params of the function. The _ref is a 
    // pointer to an VariableDeclarator where the ✨🦄 happens.
    it('Parses object destructuring param definitions.' () => {
        function toBeParsed ({it, parses, me}){}
        test(toBeParsed);
    });

    it('Parses object destructuring param definitions.' () => {
        function toBeParsed ([it, parses, me]){}
        test(toBeParsed);
    });

    // Classes while similar from an end result point of view to function
    // declarations are handled completely differently in the JS AST. 
    it('Parses a class constructor when passed through', () => {
        class ToBeParsed {
            constructor(it, parses, me) {}
        }
        test(ToBeParsed);
    });
});

Depending on what you want to use it for ES6 Proxies and destructuring may be your best bet. For example if you wanted to use it for dependency injection (using the names of the params) then you can do it as follows:

class GuiceJs {
    constructor() {
        this.modules = {}
    }
    resolve(name) {
        return this.getInjector()(this.modules[name]);
    }
    addModule(name, module) {
        this.modules[name] = module;
    }
    getInjector() {
        var container = this;

        return (klass) => {
            console.log(klass);
            var paramParser = new Proxy({}, {
                // The `get` handler is invoked whenever a get-call for
                // `injector.*` is made. We make a call to an external service
                // to actually hand back in the configured service. The proxy
                // allows us to bypass parsing the function params using
                // taditional regex or even the newer parser.
                get: (target, name) => container.resolve(name),

                // You shouldn't be able to set values on the injector.
                set: (target, name, value) => {
                    throw new Error(`Don't try to set ${name}! 😑`);
                }
            })
            return new klass(paramParser);
        }
    }
}

It's not the most advanced resolver out there but it gives an idea of how you can use a Proxy to handle it if you want to use args parser for simple DI. There is however one slight caveat in this approach. We need to use destructuring assignments instead of normal params. When we pass in the injector proxy the destructuring is the same as calling the getter on the object.

class App {
   constructor({tweeter, timeline}) {
        this.tweeter = tweeter;
        this.timeline = timeline;
    }
}

class HttpClient {}

class TwitterApi {
    constructor({client}) {
        this.client = client;
    }
}

class Timeline {
    constructor({api}) {
        this.api = api;
    }
}

class Tweeter {
    constructor({api}) {
        this.api = api;
    }
}

// Ok so now for the business end of the injector!
const di = new GuiceJs();

di.addModule('client', HttpClient);
di.addModule('api', TwitterApi);
di.addModule('tweeter', Tweeter);
di.addModule('timeline', Timeline);
di.addModule('app', App);

var app = di.resolve('app');
console.log(JSON.stringify(app, null, 4));

This outputs the following:

{
    "tweeter": {
        "api": {
            "client": {}
        }
    },
    "timeline": {
        "api": {
            "client": {}
        }
    }
}

Its wired up the entire application. The best bit is that the app is easy to test (you can just instantiate each class and pass in mocks/stubs/etc). Also if you need to swap out implementations, you can do that from a single place. All this is possible because of JS Proxy objects.

Note: There is a lot of work that would need to be done to this before it would be ready for production use but it does give an idea of what it would look like.

It's a bit late in the answer but it may help others who are thinking of the same thing. 👍

I know this is an old question, but beginners have been copypasting solutions that extract parameter names from the string representation of a function as if this was good practice in any code. Most of the time, this just hides a flaw in the logic.

Writing parameter names in the parentheses of a function declaration can be seen as a shorthand syntax for variable creation. This:

function doSomething(foo, bar) {
    console.log("does something");
}

...is akin to this:

function doSomething() {
    var foo = arguments[0];
    var bar = arguments[1];

    console.log("does something");
}

The variables themselves are stored in the function's scope, not as properties in an object. Just like you can't manipulate the name of a variable with code, there is no way to retrieve the name of a parameter as it is not a string, and it could be eliminated during JIT compilation.

I always thought of the string representation of a function as a tool for debugging purposes, especially because of this arguments array-like object. You are not required to give names to the arguments in the first place. If you try parsing a stringified function, it doesn't actually tell you about extra unnamed parameters it might take.

Here's an even worse and more common situation. If a function has more than 3 or 4 arguments, it might be logical to pass it an object instead, which is easier to work with.

function saySomething(obj) {
  if(obj.message) console.log((obj.sender || "Anon") + ": " + obj.message);
}

saySomething({sender: "user123", message: "Hello world"});

In this case, the function itself will be able to read through the object it receives and look for its properties and get both their names and values, but trying to parse the string representation of the function would only give you "obj" for parameters, which isn't useful at all.

I have read most of the answers here, and I would like to add my one-liner.

new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)').exec(Function.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '')

or

function getParameters(func) {
  return new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');
}

or for a one-liner function in ECMA6

var getParameters = func => new RegExp('(?:'+func.name+'\\s*|^)\\s*\\((.*?)\\)').exec(func.toString().replace(/\n/g, ''))[1].replace(/\/\*.*?\*\//g, '').replace(/ /g, '');

__

Let's say you have a function

function foo(abc, def, ghi, jkl) {
  //code
}

The below code will return "abc,def,ghi,jkl"

That code will also work with the setup of a function that Camilo Martin gave:

function  (  A,  b
,c      ,d
){}

Also with Bubersson's comment on Jack Allan's answer:

function(a /* fooled you)*/,b){}

__

Explanation

new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)')

This creates a Regular Expression with the new RegExp('(?:'+Function.name+'\\s*|^)\\s*\\((.*?)\\)'). I have to use new RegExp because I am injecting a variable (Function.name, the name of the function being targeted) into the RegExp.

Example If the function name is "foo" (function foo()), the RegExp will be /foo\s*\((.*?)\)/.

Function.toString().replace(/\n/g, '')

Then it converts the entire function into a string, and removes all newlines. Removing newlines helps with the function setup Camilo Martin gave.

.exec(...)[1]

This is the RegExp.prototype.exec function. It basically matches the Regular Exponent (new RegExp()) into the String (Function.toString()). Then the [1] will return the first Capture Group found in the Regular Exponent ((.*?)).

.replace(/\/\*.*?\*\//g, '').replace(/ /g, '')

This will remove every comment inside /* and */, and remove all spaces.

This also now supports reading and understanding arrow (=>) functions, such as f = (a, b) => void 0;, in which Function.toString() would return (a, b) => void 0 instead of the normal function's function f(a, b) { return void 0; }. The original regular expression would have thrown an error in its confusion, but is now accounted for.

The change was from new RegExp(Function.name+'\\s*\\((.*?)\\)') (/Function\s*\((.*?)\)/) to new RegExp('(?:'+Function.name+'\\s*|^)\\((.*?)\\)') (/(?:Function\s*|^)\((.*?)\)/)

If you want to make all the parameters into an Array instead of a String separated by commas, at the end just add .split(',').

Since JavaScript is a scripting language, I feel that its introspection should support getting function parameter names. Punting on that functionality is a violation of first principles, so I decided to explore the issue further.

That led me to this question but no built-in solutions. Which led me to this answer which explains that arguments is only deprecated outside the function, so we can no longer use myFunction.arguments or we get:

TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them

Time to roll up our sleeves and get to work:

⭐ Retrieving function parameters requires a parser because complex expressions like 4*(5/3) can be used as default values. So Gaafar's answer or James Drew's answer are so far the best approaches.

I tried the babylon and esprima parsers but unfortunately they can't parse standalone anonymous functions, as pointed out in Mateusz Charytoniuk's answer. I figured out another workaround though by surrounding the code in parentheses, so as not to change the logic:

const ast = parser.parse("(\n" + func.toString() + "\n)")

The newlines prevent issues with // (single-line comments).

⭐ If a parser is not available, the next-best option is to use a tried-and-true technique like Angular.js's dependency injector regular expressions. I combined a functional version of Computerphile's answer with humbletim's answer and added an optional ARROW boolean for controlling whether ES6 fat arrow functions are allowed by the regular expressions.

Here are two solutions I put together. Note that these have no logic to detect whether a function has valid syntax, they only extract the arguments. This is generally ok since we usually pass parsed functions to getArguments() so their syntax is already valid.

I will try to curate these solutions as best I can, but without effort from the JavaScript maintainers, this will remain an open problem.

Node.js version (not runnable until StackOverflow supports Node.js):

const parserName = 'babylon';
// const parserName = 'esprima';
const parser = require(parserName);

function getArguments(func) {
    const maybe = function (x) {
        return x || {}; // optionals support
    }

    try {
        const ast = parser.parse("(\n" + func.toString() + "\n)");
        const program = parserName == 'babylon' ? ast.program : ast;

        return program
            .body[0]
            .expression
            .params
            .map(function(node) {
                return node.name || maybe(node.left).name || '...' + maybe(node.argument).name;
            });
    } catch (e) {
        return []; // could also return null
    }
};

////////// TESTS //////////

function logArgs(func) {
    let object = {};

    object[func] = getArguments(func);

    console.log(object);
//  console.log(/*JSON.stringify(*/getArguments(func)/*)*/);
}

console.log('');
console.log('////////// MISC //////////');

logArgs((a, b) => {});
logArgs((a, b = 1) => {});
logArgs((a, b, ...args) => {});
logArgs(function(a, b, ...args) {});
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});
logArgs(async function(a, b, ...args) {});
logArgs(function async(a, b, ...args) {});

console.log('');
console.log('////////// FUNCTIONS //////////');

logArgs(function(a, b, c) {});
logArgs(function() {});
logArgs(function named(a, b, c) {});
logArgs(function(a /* = 1 */, b /* = true */) {});
logArgs(function fprintf(handle, fmt /*, ...*/) {});
logArgs(function(a, b = 1, c) {});
logArgs(function(a = 4 * (5 / 3), b) {});
// logArgs(function (a, // single-line comment xjunk) {});
// logArgs(function (a /* fooled you {});
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});
// logArgs(function ( A, b \n,c ,d \n ) \n {});
logArgs(function(a, b) {});
logArgs(function $args(func) {});
logArgs(null);
logArgs(function Object() {});

console.log('');
console.log('////////// STRINGS //////////');

logArgs('function (a,b,c) {}');
logArgs('function () {}');
logArgs('function named(a, b, c) {}');
logArgs('function (a /* = 1 */, b /* = true */) {}');
logArgs('function fprintf(handle, fmt /*, ...*/) {}');
logArgs('function( a, b = 1, c ) {}');
logArgs('function (a=4*(5/3), b) {}');
logArgs('function (a, // single-line comment xjunk) {}');
logArgs('function (a /* fooled you {}');
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');
logArgs('function ( A, b \n,c ,d \n ) \n {}');
logArgs('function (a,b) {}');
logArgs('function $args(func) {}');
logArgs('null');
logArgs('function Object() {}');

Full working example:

https://repl.it/repls/SandybrownPhonyAngles

Browser version (note that it stops at the first complex default value):

function getArguments(func) {
    const ARROW = true;
    const FUNC_ARGS = ARROW ? /^(function)?\s*[^\(]*\(\s*([^\)]*)\)/m : /^(function)\s*[^\(]*\(\s*([^\)]*)\)/m;
    const FUNC_ARG_SPLIT = /,/;
    const FUNC_ARG = /^\s*(_?)(.+?)\1\s*$/;
    const STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;

    return ((func || '').toString().replace(STRIP_COMMENTS, '').match(FUNC_ARGS) || ['', '', ''])[2]
        .split(FUNC_ARG_SPLIT)
        .map(function(arg) {
            return arg.replace(FUNC_ARG, function(all, underscore, name) {
                return name.split('=')[0].trim();
            });
        })
        .filter(String);
}

////////// TESTS //////////

function logArgs(func) {
    let object = {};

    object[func] = getArguments(func);

    console.log(object);
//  console.log(/*JSON.stringify(*/getArguments(func)/*)*/);
}

console.log('');
console.log('////////// MISC //////////');

logArgs((a, b) => {});
logArgs((a, b = 1) => {});
logArgs((a, b, ...args) => {});
logArgs(function(a, b, ...args) {});
logArgs(function(a, b = 1, c = 4 * (5 / 3), d = 2) {});
logArgs(async function(a, b, ...args) {});
logArgs(function async(a, b, ...args) {});

console.log('');
console.log('////////// FUNCTIONS //////////');

logArgs(function(a, b, c) {});
logArgs(function() {});
logArgs(function named(a, b, c) {});
logArgs(function(a /* = 1 */, b /* = true */) {});
logArgs(function fprintf(handle, fmt /*, ...*/) {});
logArgs(function(a, b = 1, c) {});
logArgs(function(a = 4 * (5 / 3), b) {});
// logArgs(function (a, // single-line comment xjunk) {});
// logArgs(function (a /* fooled you {});
// logArgs(function (a /* function() yes */, \n /* no, */b)/* omg! */ {});
// logArgs(function ( A, b \n,c ,d \n ) \n {});
logArgs(function(a, b) {});
logArgs(function $args(func) {});
logArgs(null);
logArgs(function Object() {});

console.log('');
console.log('////////// STRINGS //////////');

logArgs('function (a,b,c) {}');
logArgs('function () {}');
logArgs('function named(a, b, c) {}');
logArgs('function (a /* = 1 */, b /* = true */) {}');
logArgs('function fprintf(handle, fmt /*, ...*/) {}');
logArgs('function( a, b = 1, c ) {}');
logArgs('function (a=4*(5/3), b) {}');
logArgs('function (a, // single-line comment xjunk) {}');
logArgs('function (a /* fooled you {}');
logArgs('function (a /* function() yes */, \n /* no, */b)/* omg! */ {}');
logArgs('function ( A, b \n,c ,d \n ) \n {}');
logArgs('function (a,b) {}');
logArgs('function $args(func) {}');
logArgs('null');
logArgs('function Object() {}');

Full working example:

https://repl.it/repls/StupendousShowyOffices

The proper way to do this is to use a JS parser. Here is an example using acorn.

const acorn = require('acorn');    

function f(a, b, c) {
   // ...
}

const argNames = acorn.parse(f).body[0].params.map(x => x.name);
console.log(argNames);  // Output: [ 'a', 'b', 'c' ]

The code here finds the names of the three (formal) parameters of the function f. It does so by feeding f into acorn.parse().

You can also use "esprima" parser to avoid many issues with comments, whitespace and other things inside parameters list.

function getParameters(yourFunction) {
    var i,
        // safetyValve is necessary, because sole "function () {...}"
        // is not a valid syntax
        parsed = esprima.parse("safetyValve = " + yourFunction.toString()),
        params = parsed.body[0].expression.right.params,
        ret = [];

    for (i = 0; i < params.length; i += 1) {
        // Handle default params. Exe: function defaults(a = 0,b = 2,c = 3){}
        if (params[i].type == 'AssignmentPattern') {
            ret.push(params[i].left.name)
        } else {
            ret.push(params[i].name);
        }
    }

    return ret;
}

It works even with code like this:

getParameters(function (hello /*, foo ),* /bar* { */,world) {}); // ["hello", "world"]

(function(a,b,c){}).toString().replace(/.*\(|\).*/ig,"").split(',')

=> [ "a", "b", "c" ]

I've tried doing this before, but never found a praticial way to get it done. I ended up passing in an object instead and then looping through it.

//define like
function test(args) {
    for(var item in args) {
        alert(item);
        alert(args[item]);
    }
}

//then used like
test({
    name:"Joe",
    age:40,
    admin:bool
});

I don't know if this solution suits your problem, but it lets you redefine whatever function you want, without having to change code that uses it. Existing calls will use positioned params, while the function implementation may use "named params" (a single hash param).

I thought that you will anyway modify existing function definitions so, why not having a factory function that makes just what you want:

<!DOCTYPE html>

<html>
<head>
<meta charset="UTF-8">
<title></title>
<script type="text/javascript">
var withNamedParams = function(params, lambda) {
    return function() {
        var named = {};
        var max   = arguments.length;

        for (var i=0; i<max; i++) {
            named[params[i]] = arguments[i];
        }

        return lambda(named);
    };
};

var foo = withNamedParams(["a", "b", "c"], function(params) {
    for (var param in params) {
        alert(param + ": " + params[param]);
    }
});

foo(1, 2, 3);
</script>
</head>
<body>

</body>
</html>

Hope it helps.

Taking the answer from @jack-allan I modified the function slightly to allow ES6 default properties such as:

function( a, b = 1, c ){};

to still return [ 'a', 'b' ]

/**
 * Get the keys of the paramaters of a function.
 *
 * @param {function} method  Function to get parameter keys for
 * @return {array}
 */
var STRIP_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
var ARGUMENT_NAMES = /(?:^|,)\s*([^\s,=]+)/g;
function getFunctionParameters ( func ) {
    var fnStr = func.toString().replace(STRIP_COMMENTS, '');
    var argsList = fnStr.slice(fnStr.indexOf('(')+1, fnStr.indexOf(')'));
    var result = argsList.match( ARGUMENT_NAMES );

    if(result === null) {
        return [];
    }
    else {
        var stripped = [];
        for ( var i = 0; i < result.length; i++  ) {
            stripped.push( result[i].replace(/[\s,]/g, '') );
        }
        return stripped;
    }
}

As this has not yet been mentioned, if you are using Typescript you can emit meta-data when using Decorators which will allow you to get the parameter types.

Metadata will only be emitted if the class/function/prop has a decorator on it.
It doesn't matter which decorator.

This feature can be enabled by setting emitDecoratorMetadata to true inside tsconfig.json

{
  "compilerOptions": {
    "emitDecoratorMetadata": true
  }
}

As the metadata is still an early proposal the reflect-metadata package must be installed or Reflect.getMetadata will not be defined.

npm install reflect-metadata

You can use it as follows:

const AnyDecorator = () : MethodDecorator => {
    return target => { }
}

class Person{
    @AnyDecorator()
    sayHello(other: Person){}
}
const instance = new Person();
// This returns: Function
const funcType = Reflect.getMetadata('design:type', instance.sayHello);
// Returns an array of types, here it would be: [Person]
const funcParams = Reflect.getMetadata('design:paramtypes', instance.sayHello);

In newer versions of Angular for instance this is used to determine what to inject -> https://stackoverflow.com/a/53041387/1087372

This package uses recast in order to create an AST and then the parameter names are gathered from their, this allows it to support pattern matching, default arguments, arrow functions and other ES6 features.

https://www.npmjs.com/package/es-arguments

I don't know how to get a list of the parameters but you can do this to get how many it expects. Note this only counts arguments without a default value in the signature:

function foobar(a, b, c) {}
function foobar2(a, b=false, c=false) {}

console.log(foobar.length); // prints 3
console.log(foobar2.length); // prints 1

//See this:


// global var, naming bB
var bB = 5;

//  Dependency Injection cokntroller
var a = function(str, fn) {
  //stringify function body
  var fnStr = fn.toString();

  // Key: get form args to string
  var args = fnStr.match(/function\s*\((.*?)\)/);
  // 
  console.log(args);
  // if the form arg is 'bB', then exec it, otherwise, do nothing
  for (var i = 0; i < args.length; i++) {
    if(args[i] == 'bB') {
      fn(bB);
    }
  }
}
// will do nothing
a('sdfdfdfs,', function(some){
alert(some)
});
// will alert 5

a('sdfdsdsfdfsdfdsf,', function(bB){
alert(bB)
});

// see, this shows you how to get function args in string

The answer to this requires 3 steps:

1. To get the values of the actual parameters passed to the function (let's call it argValues). This is straight forward as it will be available as arguments inside the function.
2. To get the parameter names from the function signature (let's call it argNames). This not as easy and requires parsing the function. Instead of doing the complex regex yourself and worrying about edge cases (default parameters, comments, ...), you can use a library like babylon that will parse the function into an abstract syntax tree from which you can obtain the names of parameters.
3. The last step is to join the 2 arrays together into 1 array that has the name and value of all the parameters.

The code will be like this

const babylon = require("babylon")
function doSomething(a, b, c) {
    // get the values of passed argumenst
    const argValues = arguments

    // get the names of the arguments by parsing the function
    const ast = babylon.parse(doSomething.toString())
    const argNames =  ast.program.body[0].params.map(node => node.name)

    // join the 2 arrays, by looping over the longest of 2 arrays
    const maxLen = Math.max(argNames.length, argValues.length)
    const args = []
    for (i = 0; i < maxLen; i++) { 
       args.push({name: argNames[i], value: argValues[i]})
    }
    console.log(args)

    // implement the actual function here
}

doSomething(1, 2, 3, 4)

and the logged object will be

[
  {
    "name": "a",
    "value": 1
  },
  {
    "name": "c",
    "value": 3
  },
  {
    "value": 4
  }
]

And here's a working example https://tonicdev.com/5763eb77a945f41300f62a79/5763eb77a945f41300f62a7a

function getArgs(args) {
    var argsObj = {};

    var argList = /\(([^)]*)/.exec(args.callee)[1];
    var argCnt = 0;
    var tokens;

    while (tokens = /\s*([^,]+)/g.exec(argList)) {
        argsObj[tokens[1]] = args[argCnt++];
    }

    return argsObj;
}

Wow so many answers already.. Im pretty sure this gets buried. Even so I figured this might be useful for some.

I wasn't fully satisfied with the chosen answers as in ES6 it doesn't work well with default values. And it also does not provide the default value information. I also wanted a lightweight function that does not depend on an external lib.

This function is very useful for debugging purposes, for example: logging called function with its params, default param values and arguments.

I spent some time on this yesterday, cracking the right RegExp to solve this issue and this is what I came up with. It works very well and I'm very pleased with the outcome:

const REGEX_COMMENTS = /((\/\/.*$)|(\/\*[\s\S]*?\*\/))/mg;
const REGEX_FUNCTION_PARAMS = /(?:\s*(?:function\s*[^(]*)?\s*)((?:[^'"]|(?:(?:(['"])(?:(?:.*?[^\\]\2)|\2))))*?)\s*(?=(?:=>)|{)/m
const REGEX_PARAMETERS_VALUES = /\s*(\w+)\s*(?:=\s*((?:(?:(['"])(?:\3|(?:.*?[^\\]\3)))((\s*\+\s*)(?:(?:(['"])(?:\6|(?:.*?[^\\]\6)))|(?:[\w$]*)))*)|.*?))?\s*(?:,|$)/gm

/**
 * Retrieve a function's parameter names and default values
 * Notes:
 *  - parameters with default values will not show up in transpiler code (Babel) because the parameter is removed from the function.
 *  - does NOT support inline arrow functions as default values
 *      to clarify: ( name = "string", add = defaultAddFunction )   - is ok
 *                  ( name = "string", add = ( a )=> a + 1 )        - is NOT ok
 *  - does NOT support default string value that are appended with a non-standard ( word characters or $ ) variable name
 *      to clarify: ( name = "string" + b )         - is ok
 *                  ( name = "string" + $b )        - is ok
 *                  ( name = "string" + b + "!" )   - is ok
 *                  ( name = "string" + λ )         - is NOT ok
 * @param {function} func
 * @returns {Array} - An array of the given function's parameter [key, default value] pairs.
 */
function getParams(func) {

  let functionAsString = func.toString()
  let params = []
  let match
  functionAsString = functionAsString.replace(REGEX_COMMENTS, '')
  functionAsString = functionAsString.match(REGEX_FUNCTION_PARAMS)[1]
  if (functionAsString.charAt(0) === '(') functionAsString = functionAsString.slice(1, -1)
  while (match = REGEX_PARAMETERS_VALUES.exec(functionAsString)) params.push([match[1], match[2]])
  return params

}



// Lets run some tests!

var defaultName = 'some name'

function test1(param1, param2, param3) { return (param1) => param1 + param2 + param3 }
function test2(param1, param2 = 4 * (5 / 3), param3) {}
function test3(param1, param2 = "/root/" + defaultName + ".jpeg", param3) {}
function test4(param1, param2 = (a) => a + 1) {}

console.log(getParams(test1)) 
console.log(getParams(test2))
console.log(getParams(test3))
console.log(getParams(test4))

// [ [ 'param1', undefined ], [ 'param2', undefined ], [ 'param3', undefined ] ]
// [ [ 'param1', undefined ], [ 'param2', '4 * (5 / 3)' ], [ 'param3', undefined ] ]
// [ [ 'param1', undefined ], [ 'param2', '"/root/" + defaultName + ".jpeg"' ], [ 'param3', undefined ] ]
// [ [ 'param1', undefined ], [ 'param2', '( a' ] ]
// --> This last one fails because of the inlined arrow function!


var arrowTest1 = (a = 1) => a + 4
var arrowTest2 = a => b => a + b
var arrowTest3 = (param1 = "/" + defaultName) => { return param1 + '...' }
var arrowTest4 = (param1 = "/" + defaultName, param2 = 4, param3 = null) => { () => param3 ? param3 : param2 }

console.log(getParams(arrowTest1))
console.log(getParams(arrowTest2))
console.log(getParams(arrowTest3))
console.log(getParams(arrowTest4))

// [ [ 'a', '1' ] ]
// [ [ 'a', undefined ] ]
// [ [ 'param1', '"/" + defaultName' ] ]
// [ [ 'param1', '"/" + defaultName' ], [ 'param2', '4' ], [ 'param3', 'null' ] ]


console.log(getParams((param1) => param1 + 1))
console.log(getParams((param1 = 'default') => { return param1 + '.jpeg' }))

// [ [ 'param1', undefined ] ]
// [ [ 'param1', '\'default\'' ] ]

As you can tell some of the parameter names disappear because the Babel transpiler removes them from the function. If you would run this in the latest NodeJS it works as expected (The commented results are from NodeJS).

Another note, as stated in the comment is that is does not work with inlined arrow functions as a default value. This simply makes it far to complex to extract the values using a RegExp.

Please let me know if this was useful for you! Would love to hear some feedback!

Here's my solution -- it works for named and unnamed functions, async and non-async functions, async and non-async lambdas, and lambdas with and without parens.

const STRIP_COMMENTS = /((\/\/.*$)|(\/\*.*\*\/))/mg;
const STRIP_KEYWORDS = /(\s*async\s*|\s*function\s*)+/;
const ARGUMENT_NAMES = /\(([^)]+)\)\s*=>|([a-zA-Z_$]+)\s*=>|[a-zA-Z_$]+\(([^)]+)\)|\(([^)]+)\)/;
const ARGUMENT_SPLIT = /[ ,\n\r\t]+/;
function getParamNames(func) {
    const fnStr = func.toString()
        .replace(STRIP_COMMENTS, "")
        .replace(STRIP_KEYWORDS, "")
        .trim();
    const matches = ARGUMENT_NAMES.exec(fnStr);
    var match;
    if (matches) {
        for (var i = 1; i < matches.length; i++) {
            if (matches[i]) {
                match = matches[i];
                break;
            } 
        }
    }
    if (match === undefined) {
        return [];
    }
    return match.split(ARGUMENT_SPLIT).filter(part => part !== "");
}

Intro

So, I've seen several solutions regarding these questions:

• How to get function parameter names/values dynamically?

• Get function's default value?

• Inspect the names/values of arguments in the definition/execution of a JavaScript function

And other standalone^[1] methods, when tested with particular functions declarations, don't work or return a messy result.

These are probably unusual cases, but I wanted to propose a stand-alone solution, that covers more edge cases and also returns an object with arguments as keys paired with their default values (evaluated not strings).

Solution

/**
 * Parses the parameters of a function.toString() and returns them as an object
 * @param {Function} fn The function to get the parameters from
 * @param {*} ...args arguments to pass to the function
 * @returns {Object} An object with the parameters and their default values
*/
function fnParams(fn, ...args){

    if(typeof fn != 'function') return null;

    fn = fn.toString()

    let param = '',
        paramsList = '(', // We init the param declaration, the closing ) is added during the parsing
        paramsOBJ = '',
        comment = {block:false,line:false},
        ignoreUntil = null,
        readP = true,
        i=0,
        ch,
        whitespace = new RegExp('\\s');
    
    const LEN = fn.length;

    for(i=0;i<LEN;++i){
        ch = fn[i];
        if(comment.line){ if(ch=="\n") comment.line = false; }
        else if(comment.block){ if(ch=="/" && fn[i-1]=="*") comment.block = false; }
        else if(ch == "/" && fn[i+1] == "/") comment.line = true;
        else if(ch == "/" && fn[i+1] == "*") comment.block = true;
        else if(ch == "("){i++; break}
    }
    if(i==LEN) return null;
    while(i<LEN){
        
        ch = fn[i];
        paramsList += ch;

        if(ignoreUntil){if(ch==ignoreUntil) ignoreUntil=null;}
        else if(comment.line){ if(ch=="\n") comment.line = false; }
        else if(comment.block){ if(ch=="/" && fn[i-1]=="*") comment.block = false; }
        else if(ch == "/" && fn[i+1] == "/") comment.line = true;
        else if(ch == "/" && fn[i+1] == "*") comment.block = true;
        else if(readP){
            if(ch=="," || ch=="=" || ch==")"){
                if(param!='') paramsOBJ += param+','; 
                param='';
                if(ch=="=") readP = false
                else if(ch==")") break;
            }
            else if(ch==":") param = '';
            else if(!(  ch=='.' ||
                        whitespace.test(ch) ||
                        ch=="{" || 
                        ch=="[" || 
                        ch=="]" || 
                        ch=="}" )) param+=ch;
        }
        else{
            if(ch==",") readP = true;
            else if(ch==")") break;
            else if(ch=='"') ignoreUntil = '"';
            else if(ch=="'") ignoreUntil = "'";
            else if(ch=='`') ignoreUntil = '`';
            else if(ch=='{') ignoreUntil = '}';
            else if(ch=='[') ignoreUntil = ']';
            else if(ch=='(') ignoreUntil = ')';
        }

        i++;
    }

    return eval(`${paramsList}=>{return {${paramsOBJ}}}`).apply(this,args);
}

The solution is nothing fancy or elegant. The avarage execution time is 0.1ms ± 4%. IMO, it's a more robust solution, and actually returns the function arguments in an organized way. In the current state, it could be used to precompute the parameters.

To summarize, this solution handles everything besides functions like x=>{y} and x=>y.

Tests

let tests = [
    {
        name: "Default parameters | Basic",
        fn(x=1,y=2,z=3){
            return{
                x,
                y,
                z
            }
        },
        args:[,"PASSED",],
        this:this
    },
    {
        name: "Custom this",
        fn(
            a = this,
            b = this.special
        ){
            return{
                a,
                b
            }
        },
        args:[],
        this:{special:":P"}
    },
    {
        name: "Rest parameters",
        fn(a,b=2, ...andTheRest){
            return{
                a,
                b,
                andTheRest
            }
        },
        args:[1,,3,4,5],
        this:this
    },
    {
        name: "Default parameters | Complex",
        fn(
            a,
            b = 5+1.2-(4/2),
            c = this,
            d = Math.round(1.34*10),
            e = d,
            f = arguments,
            g = this.special,
            h = [a,b,1],
            i = {a:b,b:a}
        ){
            return{
                a,
                b,
                c,
                d,
                e,
                f,
                g,
                h,
                i
            }
        },
        args:[],
        this:{special:":P"}
    },
    {
        name: "Tricky comment | Block outside ",
        fn /*(tricked=1)*/(a=1) {
            return {
                a
            };
        },
        args:[],
        this:this
    },
    {
        name: "Tricky comment | Line outside ",
        fn //(tricked=1)
        (a=1) {
            return {
                a
            };
        },
        args:[],
        this:this
    },
    {
        name: "Tricky comment | Block inside ",
        fn (a=1,/*tricked,*/b=2) {
            return {
                a,
                b
            };
        },
        args:[],
        this:this
    },
    {
        name: "Tricky comment | Line inside ",
        fn (a=1,// tricked 
            b=2) {
            return {
                a,
                b
            };
        },
        args:[],
        this:this
    },
    {
        name: "Tricky strings",
        fn (a="1,trickA",b='2,trickB',c=`3,trickC`) {
            return {
                a,
                b,
                c
            };
        },
        args:[],
        this:this
    },
    {
        name: "Tricky delimiters",
        fn ({a="}",b='}',c=`}`}={}, [d="]"]=[]) {
            return {
                a,
                b,
                c,
                d
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Basic",
        fn([a,b,c],{d,e,f}){
            return {
                a,
                b,
                c,
                d,
                e,
                f
            }
        },
        args:[[1,2,3],{f:"<",e:"=",d:"3"}],
        this:this
    },
    {
        name: "Destructuring | Renaming",
        fn({d:rD,e:rE,f:rF}){
            return {
                rD,
                rE,
                rF
            }
        },
        args:[{f:"<",e:"=",d:"3"}],
        this:this
    },
    {
        name: "Destructuring | Further",
        fn(
            {a, b: {c: d} }
        ){
            return {
                a,
                d
            }
        },
        args:[{ a: 1, b: { c: 2 } }],
        this:this
    },
    {
        name: "Destructuring | Default",
        fn(
            [a = 1] = [],
            { b = 2 } = { b: undefined },
            { c = 3 } = { c: null },
            { d = 4 } = {},
        ){
            return {
                a,
                b,
                c,
                d
            }
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Nested",
        fn(
            [a = 1, [b = 2, c = 3, [, d]]] = [, [, -1, [, -2]]],
            { e = 1, f: g = 2, h: i , nA:{k,l,nB:{m:n,o:p=4}}} = { e: -1, h:-2, nA:{l:-3,nB:{m:-4}} },
        ) {
            return {
                a,
                b,
                c,
                d,
                e,
                g,
                i,
                k,
                l,
                n,
                p
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Binding Rest",
        fn(
            [a, b, ...{ length }] = [1, 2, 3],
            [c, d, ...[e, f, ...[g, h]]] = [1, 2, 3, 4, 5, 6],
        ) {
            return {
                a,
                b,
                length,
                c,
                d,
                e,
                f,
                g,
                h,
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Rest",
        fn(
        { a, ...b } = { a: 1, b: 2, c: 3 },
        [c, ...d] = [1, 2, 3, 4, 5, 6],
        ) {
            return {
                a,
                b,
                c,
                d
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Mixed",
        fn(
            {b:a, c:[b,c=1,{d}]} = {a:-1,b:-2,c:[-3,,{d:-4,e:-5}]}
        ) {
            return {
                a,
                b,
                c,
                d,
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Computed Property",
        fn({ ["a"]: aA, ["b"]: bB = 1 , c:{['c']:cC}} = { ["a"]: -1 , c:{['c']:-2}}) {
            return {
                aA,
                bB,
                cC
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Nested + Default",
        fn(
            { a, b:{c}={c:-2}, d:[e]=[-3]} = {a:-1}
        ) {
            return {
                a,
                c,
                e
            };
        },
        args:[],
        this:this
    },
    {
        name: "Destructuring | Alternative identifier",
        fn(
            { a, b:{c}={c:-2}, d:[e]=[-3]} = {a:-1}
        ) {
            return {
                a,
                c,
                e
            };
        },
        args:[],
        this:this
    },
]

Conclusion

Thanks to @Bergi for pointing out the issues in my previous solutions. As @Bergi (and others) said, using an already existing JS parser would be a more reliable solution.

This solution is here to offer a standalone approach.

Lastly, I don't know much about parsing, so any improvement is welcomed.

Notes

1. "standalone": By standalone here I mean: methods that don't rely on JS-Parsers (or other dependencies) to get the job done.

How I typically do it:

function name(arg1, arg2){
    var args = arguments; // array: [arg1, arg2]
    var objecArgOne = args[0].one;
}
name({one: "1", two: "2"}, "string");

You can even ref the args by the functions name like:

name.arguments;

Hope this helps!

I'll give you a short example below:

function test(arg1,arg2){
    var funcStr = test.toString()
    var leftIndex = funcStr.indexOf('(');
    var rightIndex = funcStr.indexOf(')');
    var paramStr = funcStr.substr(leftIndex+1,rightIndex-leftIndex-1);
    var params = paramStr.split(',');
    for(param of params){
        console.log(param);   // arg1,arg2
    }
}

test();

I have modified the version taken from AngularJS that implements a dependency injection mechanism to work without Angular. I have also updated the STRIP_COMMENTS regex to work with ECMA6, so it supports things like default values in the signature.

var FN_ARGS = /^\s*[^\(]*\(\s*([^\)]*)\)/m;
var FN_ARG_SPLIT = /,/;
var FN_ARG = /^\s*(_?)(.+?)\1\s*$/;
var STRIP_COMMENTS = /(\/\/.*$)|(\/\*[\s\S]*?\*\/)|(\s*=[^,\)]*(('(?:\\'|[^'\r\n])*')|("(?:\\"|[^"\r\n])*"))|(\s*=[^,\)]*))/mg;

function annotate(fn) {
  var $inject,
    fnText,
    argDecl,
    last;

  if (typeof fn == 'function') {
    if (!($inject = fn.$inject)) {
      $inject = [];
      fnText = fn.toString().replace(STRIP_COMMENTS, '');
      argDecl = fnText.match(FN_ARGS);
      argDecl[1].split(FN_ARG_SPLIT).forEach(function(arg) {
        arg.replace(FN_ARG, function(all, underscore, name) {
          $inject.push(name);
        });
      });
      fn.$inject = $inject;
    }
  } else {
    throw Error("not a function")
  }
  return $inject;
}

console.log("function(a, b)",annotate(function(a, b) {
  console.log(a, b, c, d)
}))
console.log("function(a, b = 0, /*c,*/ d)",annotate(function(a, b = 0, /*c,*/ d) {
  console.log(a, b, c, d)
}))
annotate({})

Here is another way to parse out the input parameters.

const func = (p1,p2=3,...args) => args.reduce((a,b)=>a+b-p1-p2)
const regex = /\((.*?)\)/
regex.exec(func.toString())[1].split(",")

It works by converting the function into a string and then getting only the values between the brackets.

You can access the argument values passed to a function using the "arguments" property.

    function doSomething()
    {
        var args = doSomething.arguments;
        var numArgs = args.length;
        for(var i = 0 ; i < numArgs ; i++)
        {
            console.log("arg " + (i+1) + " = " + args[i]);  
                    //console.log works with firefox + firebug
                    // you can use an alert to check in other browsers
        }
    }

    doSomething(1, '2', {A:2}, [1,2,3]);    

It's pretty easy.

At the first there is a deprecated arguments.callee — a reference to called function. At the second if you have a reference to your function you can easily get their textual representation. At the third if you calling your function as constructor you can also have a link via yourObject.constructor. NB: The first solution deprecated so if you can't to not use it you must also think about your app architecture. If you don't need exact variable names just use inside a function internal variable arguments without any magic.

https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Functions_and_function_scope/arguments/callee

All of them going to call toString and replace with re so we can create a helper:

// getting names of declared parameters
var getFunctionParams = function (func) {
    return String(func).replace(/[^\(]+\(([^\)]*)\).*/m, '$1');
}

Some examples:

// Solution 1. deprecated! don't use it!
var myPrivateFunction = function SomeFuncName (foo, bar, buz) {
    console.log(getFunctionParams(arguments.callee));
};
myPrivateFunction (1, 2);

// Solution 2.
var myFunction = function someFunc (foo, bar, buz) {
    // some code
};
var params = getFunctionParams(myFunction);
console.log(params);

// Solution 3.
var cls = function SuperKewlClass (foo, bar, buz) {
    // some code
};
var inst = new cls();
var params = getFunctionParams(inst.constructor);
console.log(params);

Enjoy with JS!

UPD: Jack Allan was provided a little bit better solution actually. GJ Jack!

Whatever the solution, it must not break on wierd functions, whose toString() looks just as wierd:

function  (  A,  b
,c      ,d
){}

Also, why use complex regular expressions? This can be done like:

function getArguments(f) {
    return f.toString().split(')',1)[0].replace(/\s/g,'').substr(9).split(',');
}

This works everywhere with every function, and the only regex is whitespace removal that doesn't even process the whole string due to the .split trick.