How to cast an object to an array? [closed]

Modern Answer

If you have an iterable, you can use the spread operator.

var a = [...o];

If you have an array-like object, (like arguments, for example,) you can get a real array made from it by calling Array.from(o).

var o = {0:"a", 1:'b', length:2};
var a = Array.from(o);

a will be ["a", "b"]. This will only work right if you have a correctly set length property.

Older Answer

If you have an array-like object, (like arguments, for example,) you can get a real array made from it by calling Array.prototype.slice.call(o).

var o = {0:"a", 1:'b', length:2};
var a = Array.prototype.slice.call(o);

a will be ["a", "b"]. This will only work right if you have a correctly set length property.

I think you are trying too hard...

It's easiest with jQuery (or similar library)

For this object:

var obj = {a: 1, b: 2, c: 3};

Arrays have a fixed key system so for the object above, you've got to throw away either the keys (a, b, c) or the values (1, 2, 3)

So either this:

var arr = $.map(obj, function (value, key) { return value; });

or this:

var arr = $.map(obj, function (value, key) { return key; });

A year ago now, but I may as well mention jQuery's makeArray function http://api.jquery.com/jQuery.makeArray/

Use a for loop for max browser compatibility.

In Javascript all arrays are objects, but not all object are arrays. Take a look at this Perfection Kills page which describes how to check that something is an Array.

To check for an array, you can use Object.prototype.toString.call(theObject). This will return [object Array] for an object that is an Array and [object Object] for an object that's not an Array (see example below):

            function myLittleTest() 
            {
                var obj, arr, armap, i;    

                  // arr is an object and an array
                arr = [1, 2, 3, 5, 7, 11]; 

                obj = {}; // obj is only an object... not an array

                alert (Object.prototype.toString.call(obj));
                  // ^ Output: [object Object]

                obj = arr; // obj is now an array and an object

                alert (Object.prototype.toString.call(arr));
                alert (Object.prototype.toString.call(obj));
                  // ^ Output for both: [object Array]

                // works in IE
                armap = [];
                for(i = 0; i < obj.length; ++i)
                {
                    armap.push(obj[i] * obj[i]);
                }

                alert (armap.join(", ")); 

            }
            // Changed from prueba();
            myLittleTest();

jsFiddle example

Among many other small utilities for manipulating objects and arrays, Underscore.js offers a toArray(obj) helper method. Documentation here: http://underscorejs.org/#toArray

It's not totally obvious from the way the documentation is written, but it works like a charm on arbitrary objects. When given an object, it iterates over the values and returns a list that contains just those values.

If you're converting an array-like object to an array you can use:

var array = Array.prototype.slice.call(arrayLike) // returns an array

Note that unlike Array.prototype.slice.call(), Array.from() is slow and always returns a dense array even if the given array-like is sparse. (In V8 it is a packed array.)

But since the question is how to cast a plain JavaScript object to an array, the answer is there is no built-in way other than to copy all own key/value pairs from the source object to a new array:

function cast2Array(value) {
    if (Array.isArray(value)) {
        return value;// value is already an array
    }
    if (value===null || value===undefined) {
        return [];// value is nullish
    }
    if (typeof value=="function" || Object(value)!==value) {
        return [value];// value is either a function or a primitive
    }
    // value is an object. Get property descriptors to apply to array
    var length, descriptors = Object.getOwnPropertyDescriptors(value);
    delete descriptors.length;// avoid potential RangeError
    try {
        length = Number(value.length);// throws if can't cast to number
    }
    catch(e) {
    }
    // Return new array with integer length >= 0 and descriptors applied
    return Object.defineProperties(Array(length > 0? length-length%1: 0), descriptors);
}

// Converts array-like objects to arrays
cast2Array(new String('test'));// ['t', 'e', 's', 't']
// Note: Unlike other methods the object doesn't need a length property
cast2Array({0:'a', 1:'b', 2:'c'});// ['a', 'b', 'c']
// or if value is already an array, no new array is created
cast2Array(value);// returns `value`
// Casting primitives simply returns an array containing the primitive
cast2Array('test');// ['test']
// or an empty array if the cast value is nullish
cast2Array(null);// []

If what you want is to cast a plain object to an iterable for use with modern features like for...of and the spread operator, use Object.entries() which takes a plain object and returns an array containing all of its own enumerable key/value pairs.

var object = {a:1, b:2, c:3};
Object.entries(object)// returns [['a', 1], ['b', 2], ['c', 3]]