Javascript Finding if Function/Class exists before calling it

Question

I know how to check to see if a property of the global context exists. Any variation of

if (typeof myFunction != 'undefined'){...}

but what if I don't know the name of the function? I think globally I could do this

if (typeof this['myFunction'] != 'undefined'){...}

but I don't know how to do that in a function like this

function load(functionName){
  if (typeof GLOBALCONTEX[functionName] != 'undefined'){
    GLOBALCONTEX[functionName](arg1 , arg2 , ...);
  }
}

And I don't want to use try/catch as I have heard it is slow.

Shaz · Accepted Answer · 2011-07-12 20:49:51Z

9

If working with a browser, substitute GLOBALCONTEX with window. Example:

function load(functionName){
  if (typeof window[functionName] != 'undefined'){
   window[functionName](arg1 , arg2 , ...);
  }
}

edited Jul 12, 2011 at 20:49

answered Jul 12, 2011 at 20:38

Shaz

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3 Comments

katspaugh Over a year ago

The question is about calling a global function using it's name as a string, that's why it's not "just as good" as just calling the function as an object.

Shaz Over a year ago

@katspaugh: That's why I removed it. ;)

puk Over a year ago

I'll accept this as the answer as it is the best response. Quick question, what percent of javascript viewings are down via a browser? 99%? 50%? more, less?

scrappedcola · Accepted Answer · 2011-07-12 20:38:30Z

0

The Globalcontext is window. All objects are attached to it.

function load(functionName){
      if (typeof window[functionName] != 'undefined'){
        window[functionName](arg1 , arg2 , ...);
      }
    }

answered Jul 12, 2011 at 20:38

scrappedcola

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1 Comment

puk Over a year ago

As a few people pointed out this is not always the case (Although it probably is 99% of the time)

Felix Kling · Accepted Answer · 2011-07-12 20:45:15Z

0

In the browser, the global object is window ^[docs]. If you use another JavaScript execution environment (like Node.js), have a look at its documentation to find out the name/reference to the global object.

Of course such a test only works for functions which are defined in global scope, not in any higher scope. So it might be that such a function is available (and accessible) but it is not in the global scope.

edited Jul 12, 2011 at 20:45

answered Jul 12, 2011 at 20:37

Felix Kling

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