How to convert a string into function in javascript?

Question

I converted a function into string using stringify and stored into database. But how to add this string as function to a variable

lets assume i get string like i am showing in A

var A = "function aF() {\n    console.log(\'change1\');\n}"

I want to add aF function to a object key like this

{handle: A }

But i am getting this in result

{ handle: 'function aF() {\n    console.log(\'change1\');\n }' }

instead i want this

{handle:[function: aF]} or {handle:[function]}

Because variable A is typeof string. Is there any way to convert A into function and then store into handle key.

It is best to rethink what it is you are trying to do such that there is no need to store code as a string. — Scott Marcus
– Scott Marcus, Commented Mar 23, 2016 at 14:32
I don't know why people giving negative point to question. People should come with solution, not with excuse don't do and all. — Rishi
– Rishi, Commented Mar 23, 2016 at 14:35
@briosheje i can't explain why i need this but i have to store function as string in database. so later i can use it. — Rishi
– Rishi, Commented Mar 23, 2016 at 14:37
@briosheje yes i am aware of this. but if the project you going to do is for your personal purpose. database is 100% safe and only personal developer will use this thing thats why i am not thinking about security and wanna do this. — Rishi
– Rishi, Commented Mar 23, 2016 at 14:51

gurvinder372 · Accepted Answer · 2016-03-24 08:52:20Z

3

You can use Function constructor to make a functions.

For Example.

var A = "function aF() {\n    console.log(\'change1\');\n}" ;
var functionStr = A.substring(A.indexOf("{")+1, A.lastIndexOf("}"));
new Function(functionStr)();

Note:

Using a string to create function object with this method is as risky as eval(). You should not do it unless you are sure that user-input is not involved. If user-input is used in making a function string then function is not considered secure as user can potentially manipulate around the authentication and authorization since system cannot control (validate) the same.

what if function aF have some parameter

You need to store the reference to the function object and invoke the same with parameter, for example

var A = "function aF() {\n    console.log(\'change1\');\n}" ;
var functionStr = A.substring(A.indexOf("{")+1, A.lastIndexOf("}"));
var functionObj = new Function(functionStr);

Now invoke this function with parameter, for example

functionObj ( args );

or use call

functionObj.call( this, args );//this is the context you want to bind to this funciton.

or use apply

functionObj.apply( this, args );//this is the context you want to bind to this funciton.

edited Mar 24, 2016 at 8:52

answered Mar 23, 2016 at 14:30

gurvinder372

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3 Comments

T.J. Crowder Over a year ago

@ScottMarcus: While that's all true, if you have to do what the OP describes (turn a string containing the function into an actual function), you're going to have to do this (or eval, or creating a script tag, or similar -- which have the same issues). The question's flaws aren't the answer's fault. :-)

Rishi Over a year ago

@gurvinder372 what if function aF have some parameter.

briosheje Over a year ago

@Rishi : Here is an example with eval + parameter jsfiddle.net/1z1sjh5n . I'm not sure how to accomplish that using the new Function prototype to be entirely honest, it looks like the closest to your solution is using this. (Also, please, next time clarify in your question that you are aware of eval problems and so on ;) )

ug_ · Accepted Answer · 2016-03-23 16:20:32Z

Heres how I would do it, this is pretty much another take on other responses but doesnt involve any sub-strings. The comments on the code pretty much say it all.

var yourExample = createFunction("function aF() {\n    console.log(\'change1\');\n}");
yourExample(); // prints change1 in console

var fnWithParam = createFunction("function aF(param1) { console.log(param1); }");
fnWithParam(2); // prints 2 in console


// creates a function from a string, that string must be a function itself.
function createFunction(fnStr) {
    // make a function which returns a function, unwrap that function by calling it with apply();
    return new Function('return ' + fnStr).apply();
}

Also to help reduce your exposure to access to objects such as window or document you can create new variables inside the function scope which creates that function. For instance:

// creates a function from a string, that string must be a function itself.
function createFunction(fnStr) {
    // make a function which returns a function, unwrap that function by calling it with apply();
    return new Function('"use strict"; var window,document; return ' + fnStr).apply();
}

Now this doesnt solve all the security issues around creating javascript from strings but I think its better than nothing.

Good readings:

"use strict"; - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

new Function() - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function

rsabir · Accepted Answer · 2016-03-23 14:41:57Z

0

An other solution would be to do :

var  A = "function aF() {\n    console.log(\'change1\');\n}" ;
eval(A);
{handle:aF}

answered Mar 23, 2016 at 14:41

rsabir

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Comments

Veniamin · Accepted Answer · 2016-03-23 15:46:04Z

Good day.

I wrote small function. It is very simple, without many validations. See here

function makeFooFromString(str){
    var m = str.match(/function\s+([^(]+?)\s*\(([^)]*)\)\s*{(.*)}/);
    // function name
    var fooname = m[1];

    // function params
    var params  = m[2];

    // function body
    var body    = m[3];

    // processing params
    if(params.length){
        params = params.split(',');
        for(var i = 0; i < params.length; i++){
            params[i] = params[i].replace(/\s*/, '');
        }
    }else{
        params = [];
    }

    params = params.join(',');

    // make our temp function body
    var text = '';
    text += 'var foo = function('+params+'){'+body+'};';
    text += 'return foo.apply(foo, arguments);';    
    return new Function(text);
};

Now i make call like this

var foo = makeFooFromString('function get_sum(a, b){ return a+b;}')

And test

console.log(foo);
console.log(foo(1, 2));
console.log(foo(3, 4));

See it on jsfiddle https://jsfiddle.net/j586xajq/

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