Converting Array-like Objects to Arrays

What are Array-like Objects?

JavaScript has “Array-like Objects”, which are Object representations of Arrays with a length property. For example:

var realArray = ['a', 'b', 'c'];
var arrayLike = {
  0: 'a',
  1: 'b',
  2: 'c',
  length: 3
};

Common examples of Array-like Objects are the arguments object in functions and HTMLCollection or NodeList objects returned from methods like document.getElementsByTagName or document.querySelectorAll.

However, one key difference between Arrays and Array-like Objects is that Array-like objects inherit from Object.prototype instead of Array.prototype. This means that Array-like Objects can’t access common Array prototype methods like forEach(), push(), map(), filter(), and slice():

var parent = document.getElementById('myDropdown');
var desiredOption = parent.querySelector('option[value="desired"]');
var domList = parent.children;

domList.indexOf(desiredOption); // Error! indexOf is not defined.
domList.forEach(function() { 
  arguments.map(/* Stuff here */) // Error! map is not defined.
}); // Error! forEach is not defined.

function func() {
  console.log(arguments);
}
func(1, 2, 3);   // → [1, 2, 3]

Convert Array-like Objects to Arrays in ES6

1. Array.from:

   const arrayLike = {
     0: 'Value 0',
     1: 'Value 1',
     length: 2
   };
   arrayLike.forEach(value => {/* Do something */}); // Errors
   const realArray = Array.from(arrayLike);
   realArray.forEach(value => {/* Do something */}); // Works
   
2. for...of:

   var realArray = [];
   for(const element of arrayLike) {
     realArray.append(element);
   }
3. Spread operator:

   [...arrayLike]
4. Object.values:

   var realArray = Object.values(arrayLike);
5. Object.keys:

   var realArray = Object
      .keys(arrayLike)
      .map((key) => arrayLike[key]);

Convert Array-like Objects to Arrays in ≤ ES5

Use Array.prototype.slice like so:

var arrayLike = {
  0: 'Value 0',
  1: 'Value 1',
  length: 2
};
var realArray = Array.prototype.slice.call(arrayLike);
realArray = [].slice.call(arrayLike); // Shorter version

realArray.indexOf('Value 1'); // Wow! this works

You can also use Function.prototype.call to call Array.prototype methods on Array-like objects directly, without converting them:

var domList = document.querySelectorAll('#myDropdown option');

domList.forEach(function() { 
  // Do stuff
}); // Error! forEach is not defined.

Array.prototype.forEach.call(domList, function() { 
  // Do stuff
}); // Wow! this works

You can also use [].method.bind( arrayLikeObject ) to borrow array methods and glom them on to your object:

var arrayLike = {
  0: 'Value 0',
  1: 'Value 1',
  length: 2
};

arrayLike.forEach(function() {
  // Do stuff
}); // Error! forEach is not defined.

[].forEach.bind(arrayLike)(function(val){
  // Do stuff with val
}); // Wow! this works

Modifying Items During Conversion

In ES6, while using Array.from, we can specify a map function that returns a mapped value for the new array being created.

Array.from(domList, element => element.tagName); // Creates an array of tagName's

See Arrays are Objects for a detailed analysis.