# How Arguments work in JavaScript - JavaScript Arguments Guide

**URL:** <https://forum.freecodecamp.org/t/how-arguments-work-in-javascript-javascript-arguments-guide/14283>\
**Category:** freeCodeCamp Contributors\
**Tags:** guide\
**Created:** [June 25, 2016, 3:50am UTC](https://forum.freecodecamp.org/t/how-arguments-work-in-javascript-javascript-arguments-guide/14283 "2016-06-25T03:50:43Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![camperbot](https://sea3.discourse-cdn.com/freecodecamp/user_avatar/forum.freecodecamp.org/camperbot/32/18364_2.png) [@camperbot](https://forum.freecodecamp.org/u/camperbot)\
**Post date:** [June 25, 2016, 3:50am UTC](https://forum.freecodecamp.org/t/how-arguments-work-in-javascript-javascript-arguments-guide/14283/1 "2016-06-25T03:50:43Z")

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The arguments object is an object that stores all of the values passed to the function. Python developers might think of JS object as a dictionary. The arguments object takes on the structure of a JSON Object.  
Try the following code:

```auto
 function viewArgs() {
    return arguments;
}
console.log(viewArgs([3, 5, 1, 2, 2], 2, 3, 5));    
console.log(viewArgs([2, 3, 2, 3, 2, 3]));   
console.log(viewArgs(3,2,1,"life the universe and all"));
console.log(viewArgs("Douglas","Adams"));
console.log(viewArgs(["hello", "World!", 'and', 'thanks', 'for','all','the', 'fish'], "dolphines", 42));    

```

returns the following:

```auto
{ '0': [3, 5, 1, 2, 2], '1': 2, '2': 3, '3': 5 }
{ '0': [2, 3, 2, 3, 2, 3] }
{ '0': 3, '1': 2, '2': 1, '3': 'life the universe and all' }
{ '0': 'Douglas', '1': 'Adams' }
{ '0': ['hello', 'World!', 'and', 'thanks', 'for', 'all', 'the', 'fish'], '1': 'dolphins', '2': 42 }

```

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.

- 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 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):

```javascript
var args = Array.prototype.slice.call(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:

```javascript
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 in 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_**

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:

```javascript
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](https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments)

---

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**Author:** ![kevcomedia](https://sea3.discourse-cdn.com/freecodecamp/user_avatar/forum.freecodecamp.org/kevcomedia/32/125737_2.png) [@kevcomedia](https://forum.freecodecamp.org/u/kevcomedia)\
**Post date:** [August 7, 2017, 1:47am UTC](https://forum.freecodecamp.org/t/how-arguments-work-in-javascript-javascript-arguments-guide/14283/2 "2017-08-07T01:47:12Z")

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