Javascript if function check

Question

I have searched and found numerous discussions on this but none of the solutions seem to work.

Scenario: I have a central script which processes a complex calculator. This calculator has over a 1000 separate calculators, each of which is called via a function.

Each of those functions is loaded dynamically based on user selection (keep overhead down etc...)

So, core script is similar to this:

var m1 = 5;
var m2 = 8;


dynamicfunction1();
dynamicfunction2();

var m3 = m1 + m2;

The dynamic functions can change the values of m1 for example, depending on criteria (I'm keeping the example short).

So, I want to first check if the function exists before I run it, if not, ignore it.

So, I intended to have:

FncChk("dynamicfunction1");
FncChk("dynamicfunction2");

Below is an example where I have created one function, but tried to test 2 (clearly 1 doesn't exist). The issue comes around window[] (applied rather than eval(); )I just don't seem to be able to get the string to translate into a function name to execute it.

function MonkeysEatBananas() {
    alert("Function triggered, Monkeys are eating bananas");
}

function FncChk(FuncName) {
    var fn = window[FuncName];
    alert(fn + "---> " + FuncName);

    if (typeof fn == "function" ){alert("Run function hetttre");}else{alert("Function "+FuncName+" not loaded");}
    }
}



FncChk("MinionsEatBananas");

FncChk("MonkeysEatBananas");

I can validate the function as a function by using an inline statement like:

if(  typeof MonkeysEatBananas == "function" ){alert("Run function here");}else{alert("Function "+FuncName+" not loaded");}

but not when I wrap it in a function for wider user and less code. I just know I'm doing something stupid but I've killed hours on this, hence time to fire a flare! Any kicks in the right direction appreciated.

Don't do this all in the global window space. Use properties of an object literal — charlietfl
– charlietfl, Commented Jul 17, 2017 at 14:47
There are close to 800000 lines of calculation. over 1000 calculators — Pete - iCalculator
– Pete - iCalculator, Commented Jul 17, 2017 at 15:01
Your code appears to work fine. Have a look at this jsfiddle.net/rcondrnt and see what I've not done that you're doing. — fdomn-m
– fdomn-m, Commented Jul 17, 2017 at 15:39

fdomn-m · Accepted Answer · 2017-07-17 20:26:02Z

You can use window["functionName"] directly, as long as the function is global.

To test if a function, use typeof window["functionName"] == "function" and to execute, just add ().

function abc() { console.log("abc")}

function runFunc(f)
{
  var fn = window[f];
  //console.log(fn)
  if (typeof fn == "function") fn(); else console.log(f + " not found")
}

runFunc("abc");
runFunc("def");

The jquery version is pretty much the same:

function abc() { console.log("abc")}

function runFunc(f)
{
  var fn = window[f];
  if ($.isFunction(fn)) fn(); else console.log(f + " not found")
}

runFunc("abc");
runFunc("def");

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

If you use namespaces, you can do the same but just using the object instead of window (depending on how you implement namespaces, basic object example below):

var calc = {};
calc.abc = function() { console.log("abc") }

calc.runFunc = function(f) {
  var fn = calc[f];
  if (typeof fn === "function") fn();
  else console.log(f + " not found")
}

calc.runFunc("abc");
calc.runFunc("def");

// global namespace (window) now only has 'calc' and not all the calc functions
console.log("window[abc] = " + window["abc"]);

// can also call directly
calc.abc();

Here's an alternative namespacing method using a self-executing function (or, if you're a stickler: Immediately-Invoked Function Expression (IIFE))

var calc = (function() {

  var publicScope = {};

  //Public method
  publicScope.runFunc = function(f) {
    var fn = methods[f];
    if (typeof fn === "function") fn();
    else console.log(f + " not found")    
  };

  var methods = {}; 

  //Private method
  methods.abc = function () {
    console.log("abc");
  }

  //Return only the public parts
  return publicScope;
}());

calc.runFunc("abc");
calc.runFunc("def");

//cannot call directly (as they're private) (gives error)
calc.methods.abc();

Thanks @freedom-m That is a great explanation. I hope this thread who helps others, your answer is very comprehensive. During searches all I could find were the odd one liners etc. rather than an actual working solution. kudos :)

Jonas Wilms · Accepted Answer · 2017-07-17 14:44:26Z

1

You could use the in operator:

if("func" in window) func();

and you may have a look at OOP, and dynamic function names are always a bad idea...

answered Jul 17, 2017 at 14:44

Jonas Wilms

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1 Comment

Pete - iCalculator Over a year ago

Thanks Jonas, that didn't work. I'm looking to validate if it is a function within a simple function name. So function IsItFunction(somefunctionnamecheck){ IF yes, execute, IF not function ignore }

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