JavaScript variable seems to be treated as function

Question

I'm developing some JavaScript that will sit on a SharePoint page. SharePoint provides a global function getCurrentCtx(). Unfortunately this function is only available on certain pages. My JavaScript needs to test to see if this function exists.

Easy, right?

if(getCurrentCtx) {
    //Code here
}

Not so fast:

Uncaught ReferenceError: getCurrentCtx is not defined

WTF? If it's not defined, it should be undefined which is falsey and so the if statement should simply be skipped.

console.log(getCurrentCtx)

Uncaught ReferenceError: getCurrentCtx is not defined

As far as I know, the uncaught referencerror exception occurs when you try to call a function that doesn't exist. So why am I getting it when I'm just trying to retrieve the value of a variable?

Thanks,

YM

This [answer](http://stackoverflow.com/a/23370622/2055998) provides the solution. — PM 77-1, Aug 05 '14 at 23:37

Felix Kling · Answer 1 · 2014-08-05T23:36:39.020

3

As far as I know, the uncaught referencerror exception occurs when you try to call a function that doesn't exist.

Incorrect.

If you are trying read the value of any variable (or more general, binding) that doesn't exist (e.g. no var getCurrentCtx anywhere), then JS throws a reference error.

Relevant part in the specification.

Exception: typeof getCurrentCtx would return "undefined", because it tests whether the variable (reference) is resolvable (exists) before it reads it.

edited Aug 05 '14 at 23:36

answered Aug 05 '14 at 23:30

Felix Kling

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Variables at don't exist are undeclared. (And since `typeof` doesn't need the variable to be declared, the OP can use it in their `if` statement). – Quentin Aug 05 '14 at 23:34
Oh interesting, I never knew that. Thanks! – Joshua Walsh Aug 05 '14 at 23:43

score 1 · Accepted Answer · answered Aug 05 '14 at 23:32

1

Variables that are not declared throw the Uncaught ReferenceError exception. Undefined properties return undefined. Something like this will probably work.

if(typeof getCurrentCtx !== "undefined") {
    //Code here
}

Alternately, the following might also work.

if(self.getCurrentCtx) {
    //Code here
}

Both of these are untested on Sharepoint, but would work in plain JavaScript.

answered Aug 05 '14 at 23:32

Alexander O'Mara

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I'm giving this one the green tick because it mentons the distinction between variables and properties. Attempting to access the variable as a property of the window object is easier for me than doing a typeof. – Joshua Walsh Aug 05 '14 at 23:44

JavaScript variable seems to be treated as function

2 Answers2