javascript - "object is not a function" when saving function.call to a variable - Stack Overflow

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I was trying to make my code smaller by caching functions to variables. For example:

function test(){
   var a = Array.prototype.slice,
   b = a.call(arguments);
   // Do something
   setTimeout(function(){
     var c = a.call(arguments);
     // Do something else
   }, 200);
}

So instead of calling Array.prototype.slice.call(arguments), I can just do a.call(arguments);.

I was trying to make this even smaller by caching Array.prototype.slice.call, but that wasn't working.

function test(){
   var a = Array.prototype.slice.call,
   b = a(arguments);
   // Do something
   setTimeout(function(){
     var c = a(arguments);
     // Do something else
   }, 200);
}

This gives me TypeError: object is not a function. Why is that?

typeof Array.prototype.slice.call returns "function", like expected.

Why can't I save .call to a variable (and then call it)?

I was trying to make my code smaller by caching functions to variables. For example:

function test(){
   var a = Array.prototype.slice,
   b = a.call(arguments);
   // Do something
   setTimeout(function(){
     var c = a.call(arguments);
     // Do something else
   }, 200);
}

So instead of calling Array.prototype.slice.call(arguments), I can just do a.call(arguments);.

I was trying to make this even smaller by caching Array.prototype.slice.call, but that wasn't working.

function test(){
   var a = Array.prototype.slice.call,
   b = a(arguments);
   // Do something
   setTimeout(function(){
     var c = a(arguments);
     // Do something else
   }, 200);
}

This gives me TypeError: object is not a function. Why is that?

typeof Array.prototype.slice.call returns "function", like expected.

Why can't I save .call to a variable (and then call it)?

• javascript

Share Improve this question Follow asked Dec 16, 2011 at 16:07 gen_Ericgen_Eric 227k4242 gold badges303303 silver badges342342 bronze badges 2

• which javascript engine gave you this error? – Dan D. Commented Dec 16, 2011 at 16:12
• possible duplicate of How do you reference Array.prototype.slice.call()? – pimvdb Commented Dec 16, 2011 at 16:13

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3 Answers 3

Sorted by: Reset to default 7

Function.prototype.call is an ordinary function that operates on the function passed as this.

When you call call from a variable, this bees window, which is not a function.
You need to write call.call(slice, someArray, arg1, arg2)

Try this:

function test(){
   var a = function(args){
       return Array.prototype.slice.call(args);
   };
   b = a(arguments);
   // Do something
   setTimeout(function(){
     var c = a(arguments);
     // Do something else
   }, 200);
}

The same error will happen if you try doing something like:

var log = console.log;
log("Hello");

The reason is that when you do this you are assigning the function x (in my example log) to the variable log. BUT the function contains a call to this which now refers to window and not to console, which then throws an error that this is not an object

The problem is that call is a method (a function that belongs to an object) that expects its owner (its this) to be a function. When you write a = Array.prototype.slice.call, you're copying the function, but not the owner.

The "object is not a function" message isn't saying that a isn't a function, it's saying that its this isn't a function. You could technically achieve what you describe by writing a.call(Array.prototype.slice, arguments), but obviously that's not what you want!

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