How can I verify if a function was defined in JavaScript?

Question

How can I verify if a function was defined already?

Will this work?

if(window.opener.MyFunctionBlah) {  ...  }

gehsekky · Accepted Answer · 2017-03-08 21:07:34Z

6

if (typeof yourFunctionName === 'function') {
    yourFunctionName();
}

edited Mar 8, 2017 at 21:07

answered May 25, 2009 at 19:20

gehsekky

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4 Comments

I.devries Over a year ago

If i remember correctly, (typeof function(){}) is 'object' in IE.

ajm Over a year ago

Nope, it's "function" as well. I think you're thinking of typeof for arrays.

Sorin Mocanu Over a year ago

In this specific case it may be "if (window.opener && typeof window.opener.MyFunctionBlah == 'function')". You can't expect window.opener to be always an object...

David Padbury Over a year ago

In the case of window.opener even if MyFunctionBlar is a function. In IE typeof window.opener.MyFunctionBlar will be object.

Marc W · Accepted Answer · 2009-05-25 19:19:32Z

2

Yes. Functions are just properties of objects like any other, so you can treat them as such. The conditional you posted above will return true if that function (or any member of window.opener called MyFunctionBlah) is defined and non-null.

answered May 25, 2009 at 19:19

Marc W

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1 Comment

Matthew Crumley Over a year ago

Technically, non-null, non-false, non-zero, and non-empty-string... but hopefully, any name that looks like a function would actually be a function if it exists.

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