How do you reference Array.prototype.slice.call()?

Question

I am writing a script in which I need to clone arrays in many different places. For this reason, I would like to do the following to emulate a cloning function:

var clone = [].slice.call;
var arr1 = [1,2,3,4,5,6,7,8,9,10];
var arr2 = clone(arr1, 0);

Unfortunately, the above code results in: TypeError: object is not a function. I realize there are many functions out there to do deep cloning and shallow copies but I just want to use the built in method. Interestingly enough, the following does work:

var clone = [].slice;
var arr1 = [1,2,3,4,5,6,7,8,9,10];
var arr2 = clone.call(arr1, 0);

Does anyone know why the first block doesn't work while the second does? Is there any way to reference a functions call and apply functions without throwing errors when calling the referenced function?

Answer 1

I have to definitely agree with both Felix King and pimvdb. I think the only drawback to using the Function.protoytpe.bind() function is the fact that this is not a function that is available in all browsers (IE6 for example). An alternative would be to use a JavaScript library that provides the curry() function. Another alternative would be to define a function which gives you the ability to retrieve the call function for any other function. Here is a definition that I posted on my blog for such a function which I called getCall():

Function.prototype.getCall = function() {
  var realFn = this;
  return function(objThis) {
    return realFn.apply(objThis, Array.prototype.slice.call(arguments, 1));
  };
};

Now, with this definition, you could do the following to get a reference to the call function of the slice function:

var slice = [].slice.getCall();

Answer 2

Sweet and simple:

slice = Function.prototype.call.bind(Array.prototype.slice);
slice([1,2,3]); // [1,2,3]
slice({length:3,0:1,1:2,2:3}); // [1,2,3]

Answer 3

The problem is that whatever_function.call is equal to Function.prototype.call. Thus, you effectively save a reference to Function.prototype.call and the information that it is the slice function is lost.

Compare it with a custom function:

Function.prototype.custom = function() { console.log(this) };

[].slice.custom(); // logs slice function

var ref = [].slice.custom;
ref(); // logs window object

A method of keeping the this value from being changed is using Function.prototype.bind:

var ref = [].slice.call.bind([].slice);

Now,

ref([1,2,3], 1); // [2, 3]

because when calling the .call function, the this value is bound to the slice function and everything works as expected.

Answer 4

You can clone an array by calling slice directly:

var arr2 = arr1.slice();

If you want a clone function, you can do:

var clone = function(arr) { return arr.slice(); };

If you really want to prototype function (which is not necessary as long as the function is not overwritten):

var clone = function(arr) { return [].slice.call(arr); };

---

Why can't you reference call or apply directly?

It does not work for the same reason assigning a method of an object to a variable does not "work".

If you call func.call() then this inside call will be a reference to func, a function object.

If you assign call to a variable then the context is lost. You have a reference to the generic call function. Thus you'd have to pass the correct context (the method you want to apply call to) as first parameter to call again:

var clone = [].slice.call;
var arr2 = clone.call([].slice, arr1);

This is not really an improvement and quite confusing.

call and apply are methods that every functions inherits from Function.prototype. Functions don't have their own version of them. [].slice.call === [].splice.call yields true.

Answer 5

The difference is the scope of the function, i.e. what "this" is. I'm not sure what the correct technical terms are, but the "this" is not the same when a function is called "stand alone" or as a property of an object.

var myObj = {};
myObj.foo = function () {
    console.log(this.bar);
};
myObj.bar = 1234;

var fooRef = myObj.foo;

myObj.foo(); // 1234
fooRef(); // undefined

You can however create a function that wraps a call to the function and passes on all the arguments:

var fooEncapsulated = function () {
    return myObj.foo.apply(myObj, arguments);
}

fooEncapsulated(); // 1234

For the record, the most common way of doing this is:

Array.prototype.slice.call(myArray, other, arguments, here);