freeCodeCamp/guide/english/javascript/arguments/index.md

title
Arguments

The arguments object is an array-like object (in that the structure of the object is similar to that of an array however it should not be considered an array as it has all the functionality of an object) that stores all of the arguments that you passed to a function and is proprietary to that function in particular. If you were to pass 3 arguments to a function, say storeNames(), those 3 arguments would be stored inside an object called arguments and it would look like this when we pass the arguments storeNames("Mulder", "Scully", "Alex Krycek") to our function:

• First, we declare a function and make it return the arguments object.

function storeNames() { return arguments; }

• Then, when we execute that function with n arguments, 3 in this case, it will return the object to us and it will look like an array. We can convert it to an array, but more on that later...

// If we execute the following line in the console:
storeNames("Mulder", "Scully", "Alex Kryceck");
// The output will be { '0': 'Mulder', '1': 'Scully', '2': 'Alex Kryceck' }

If you want to know more about this, such as converting it to an array or the optimization problem that comes with using the slice() method and how to solve it, click on read more (Gitter Chat Only).

Treat it as an array

You can invoke arguments by using arguments[n] (where n is the index of the argument in the array-like object) but if you want to use it as an array for iteration purposes or applying array methods to it, you need to convert it to an array by declaring a variable and using the Array.prototype.slice.call method (because arguments is not an array):

var args = Array.prototype.slice.call(arguments);

// or the es6 way:
var args = Array.from(arguments)

Since slice() has two (the parameter end is optional) parameters, you can grab a certain portion of the arguments by specifying (using the slice.call() method renders these two parameters optional, not just end) the beginning and the ending of your portion; check out the following code:

function getGrades() {
    var args = Array.prototype.slice.call(arguments, 1, 3);
    return args;
}

// Let's output this!
console.log(getGrades(90, 100, 75, 40, 89, 95));

// OUTPUT SHOULD BE: //
// [100, 75] <- Why? Because it started from index 1 and stopped at index 3
// so, index 3 (40) wasn't taken into consideration.
//
// If we remove the '3' parameter, leaving just (arguments, 1) we'd get
// every argument from index 1: [100, 75, 40, 89, 95].

Optimization issues with Array.slice()

There is a little problem, it's not recommended to use slice in the arguments object (optimization reasons)...

Important: You should not slice on arguments because it prevents optimizations in JavaScript engines (V8 for example). Instead, try constructing a new array by iterating through the arguments object.

by Mozilla Developer Network (reference)

So, what other method is available to convert arguments to an array? I recommend the for-loop (not the for-in loop), you can do it like this:

var args = []; // Empty array, at first.
for (var i = 0; i < arguments.length; i++) {
    args.push(arguments[i])
} // Now 'args' is an array that holds your arguments.

For more information on the optimization issues:
Optimization Killers: Managing Arguments

ES6 rest parameter as a way to circumvent the arguments object

In ES2015/ES6 it is possible to use the rest parameter (...) instead of the arguments object in most places. Say we have the following function (non-ES6):

function getIntoAnArgument() {
    var args = arguments.slice();
    args.forEach(function(arg) {
        console.log(arg);
    });
}

That function can be replaced in ES6 by:

function getIntoAnArgument(...args) {
    args.forEach(arg => console.log(arg));
}

note that we also used an arrow function to shorten the forEach callback!

The arguments object is not available inside the body of an arrow function.

The rest parameter must always come as the last argument in your function definition.
function getIntoAnArgument(arg1, arg2, arg3, ...restOfArgs /*no more arguments allowed here*/) { //function body }