Why use Array.prototype.slice.call with arguments

Question

I am calling a method using apply and I do not know how many arguments I will be passing:

At the moment my code looks like this:

selectFolder: function(e){
  e.preventDefault();

  this.addSelectedClass.apply(this, Array.prototype.slice.call(arguments));
},

The only reason I am using Array.prototype.slice is because it is in most examples.

Why would I not just pass arguments like this:

 this.addSelectedClass.apply(this, arguments);

Answer 1

arguments with apply() are fine

When calling apply on a function it's ok to just use original arguments. SO in your example you can easily replace the line with this one:

this.addSelectedClass.apply(this, arguments);

arguments with call() are not fine

But if you'd be calling call then you should convert arguments to an actual array (which is done by a slice() call) if you'd want to pass your function an array of arbitrary objects/values. That's why call() is usually used when you know exactly the number of arguments you'd want to pass and pass them individually.

arguments are not an actual array object. They seem to be (have length property for instance), but they're not.

Arguments on Mozilla Deveveloper Network

A bit of explanation to make it completely clear

Documentation related to apply() and call() looks like this:

func.apply(thisArg[, argsArray]);
func.call(thisArg[, arg1[, arg2[, ...]]]);

We may execute the same function using any of these.

1. When using apply() we can pass all arguments at once. slice() is usually used to omit a particular argument from the call or to convert arguments to actual array.

2. When using call() we pass individual arguments and as many as we need to. If we'd provide arguments converted to an array, our called function would get one parameter of type array with all arguments supplied in that particular array.

I hope this clears thing up even more.

Answer 2

Because arguments is not an array. Running Array.prototype.slice makes it a normal array with functions like map and forEach. Basically anything in Array.prototype. arguments is, without this, just an object with a length property.

In your particular case, passing just arguments would probably work.