arrays

javascript

object

What is the best way to convert:

['a','b','c']

to:

{
  0: 'a',
  1: 'b',
  2: 'c'
}

Solution 1

ECMAScript 6 introduces the easily polyfillable Object.assign:

The Object.assign() method is used to copy the values of all enumerable own properties from one or more source objects to a target object. It will return the target object.

Object.assign({}, ['a','b','c']); // {0:"a", 1:"b", 2:"c"}

The own length property of the array is not copied because it isn't enumerable.

Also, you can use ES8 spread syntax on objects to achieve the same result:

{ ...['a', 'b', 'c'] }

For custom keys you can use reduce:

['a', 'b', 'c'].reduce((a, v) => ({ ...a, [v]: v}), {}) 
// { a: "a", b: "b", c: "c" }

Solution 2

With a function like this:

function toObject(arr) {
  var rv = {};
  for (var i = 0; i < arr.length; ++i)
    rv[i] = arr[i];
  return rv;
}

Your array already is more-or-less just an object, but arrays do have some "interesting" and special behavior with respect to integer-named properties. The above will give you a plain object.

edit oh also you might want to account for "holes" in the array:

function toObject(arr) {
  var rv = {};
  for (var i = 0; i < arr.length; ++i)
    if (arr[i] !== undefined) rv[i] = arr[i];
  return rv;
}

In modern JavaScript runtimes, you can use the .reduce() method:

var obj = arr.reduce(function(acc, cur, i) {
  acc[i] = cur;
  return acc;
}, {});

That one also avoids "holes" in the array, because that's how .reduce() works.

Solution 3

You could use an accumulator aka reduce.

['a','b','c'].reduce(function(result, item, index, array) {
  result[index] = item; //a, b, c
  return result;
}, {}) //watch out the empty {}, which is passed as "result"

Pass an empty object {} as a starting point; then "augment" that object incrementally. At the end of the iterations, result will be {"0": "a", "1": "b", "2": "c"}

If your array is a set of key-value pair objects:

[{ a: 1},{ b: 2},{ c: 3}].reduce(function(result, item) {
  var key = Object.keys(item)[0]; //first property: a, b, c
  result[key] = item[key];
  return result;
}, {});

will produce: {a: 1, b: 2, c: 3}

For the sake of completeness, reduceRight allows you to iterate over your array in reverse order:

[{ a: 1},{ b: 2},{ c: 3}].reduceRight(/* same implementation as above */)

will produce: {c:3, b:2, a:1}

Your accumulator can be of any type for you specific purpose. For example in order to swap the key and value of your object in an array, pass []:

[{ a: 1},{ b: 2},{ c: 3}].reduce(function(result, item, index) {
  var key = Object.keys(item)[0]; //first property: a, b, c
  var value = item[key];
  var obj = {};
  obj[value] = key;
  result.push(obj);
  return result;
}, []); //an empty array

will produce: [{1: "a"}, {2: "b"}, {3: "c"}]

Unlike map, reduce may not be used as a 1-1 mapping. You have full control over the items you want to include or exclude. Therefore reduce allows you to achieve what filter does, which makes reduce very versatile:

[{ a: 1},{ b: 2},{ c: 3}].reduce(function(result, item, index) {
  if(index !== 0) { //skip the first item
    result.push(item);
  }
  return result;
}, []); //an empty array

will produce: [{2: "b"}, {3: "c"}]

Caution: reduce and Object.key are part of ECMA 5th edition; you should provide a polyfill for browsers that don't support them (notably IE8).

See a default implementation by Mozilla.

Solution 4

If you're using jquery:

$.extend({}, ['a', 'b', 'c']);

Solution 5

For completeness, ECMAScript 2015(ES6) spreading. Will require either a transpiler(Babel) or an environment running at least ES6.

console.log(
   { ...['a', 'b', 'c'] }
)

Solution 6

I'd probably write it this way (since very rarely I'll not be having the underscorejs library at hand):

var _ = require('underscore');

var a = [ 'a', 'b', 'c' ];
var obj = _.extend({}, a);
console.log(obj);
// prints { '0': 'a', '1': 'b', '2': 'c' }

Solution 7

In case you want to use one of the properties of the iterated objects as key, for example:

// from:
const arr = [
    {
        sid: 123,
        name: 'aaa'
    },
    {
        sid: 456,
        name: 'bbb'
    },
    {
        sid: 789,
        name: 'ccc'
    }
];
// to:
{
  '123': { sid: 123, name: 'aaa' },
  '456': { sid: 456, name: 'bbb' },
  '789': { sid: 789, name: 'ccc' }
}

Use:

const result = arr.reduce((obj, cur) => ({...obj, [cur.sid]: cur}), {})

Solution 8

we can use Object.assign and array.reduce function to convert an Array to Object.

var arr = [{a:{b:1}},{c:{d:2}}] 
var newObj = arr.reduce((a, b) => Object.assign(a, b), {})

console.log(newObj)

Solution 9

Here is an O(1) ES2015 method just for completeness.

var arr = [1, 2, 3, 4, 5]; // array, already an object
Object.setPrototypeOf(arr, Object.prototype); // now no longer an array, still an object

Solution 10

FWIW, one another recent approach is to use the new Object.fromEntries along with Object.entries as follows:

const arr = ['a','b','c'];
arr[-2] = 'd';
arr.hello = 'e';
arr.length = 17;
const obj = Object.fromEntries(Object.entries(arr));

...which allows for avoiding storing sparse array items as undefined or null and preserves non-index (e.g., non-positive-integer/non-numeric) keys.

{ 0: "a", 1: "b", 2: "c", "-2": "d", hello: "e" }

(Same result here as with @Paul Draper's Object.assign answer.)

One may wish to add arr.length, however, as that is not included:

obj.length = arr.length;

Solution 11

Using javascript#forEach one can do this

var result = {},
    attributes = ['a', 'b','c'];

attributes.forEach(function(prop,index) {
  result[index] = prop;
});

With ECMA6:

attributes.forEach((prop,index)=>result[index] = prop);

Solution 12

If you're using ES6, you can use Object.assign and the spread operator

{ ...['a', 'b', 'c'] }

If you have nested array like

var arr=[[1,2,3,4]]
Object.assign(...arr.map(d => ({[d[0]]: d[1]})))

Solution 13

A quick and dirty one:

var obj = {},
  arr = ['a','b','c'],
  l = arr.length; 

while( l && (obj[--l] = arr.pop() ) ){};

Solution 14

Quick and dirty #2:

var i = 0
  , s = {}
  , a = ['A', 'B', 'C'];

while( i < a.length ) { s[i] = a[i++] };

Solution 15

More browser supported and more flexible way of doing that is using a normal loop, something like:

const arr = ['a', 'b', 'c'],
obj = {};

for (let i=0; i<arr.length; i++) {
   obj[i] = arr[i];
}

But also the modern way could be using the spread operator, like:

{...arr}

Or Object assign:

Object.assign({}, ['a', 'b', 'c']);

Both will return:

{0: "a", 1: "b", 2: "c"}

Solution 16

A simple and cheeky method of quickly converting an Array of items in to an Object

function arrayToObject( srcArray ){
    return  JSON.parse( JSON.stringify( srcArray ) );
}

Then using it like so...

var p = [0,2,3,'pork','pie',6];
obj = new arrayToObject( p );
console.log( obj[3], obj[4] )
// expecting `pork pie`

Output:

pork pie

Checking the type:

typeof obj
"object"

AND things wouldn't be complete if there wasn't a prototype method

Array.prototype.toObject =function(){
    return  JSON.parse( JSON.stringify( this ) );
}

Using like:

var q = [0,2,3,'cheese','whizz',6];
obj = q.toObject();
console.log( obj[3], obj[4] )
// expecting `cheese whizz`

Output:

cheese whizz

*NOTE that there is no naming routine, so if you want to have specific names, then you will need to continue using the existing methods below.


Older method

This allows you to generate from an array an object with keys you define in the order you want them.

Array.prototype.toObject = function(keys){
    var obj = {};
    var tmp = this; // we want the original array intact.
    if(keys.length == this.length){
        var c = this.length-1;
        while( c>=0 ){
            obj[ keys[ c ] ] = tmp[c];
            c--;
        }
    }
    return obj;
};

result = ["cheese","paint",14,8].toObject([0,"onion",4,99]);

console.log(">>> :" + result.onion); will output "paint", the function has to have arrays of equal length or you get an empty object.

Here is an updated method

Array.prototype.toObject = function(keys){
    var obj = {};
    if( keys.length == this.length)
        while( keys.length )
            obj[ keys.pop() ] = this[ keys.length ];
    return obj;
};

Solution 17

.reduce((o,v,i)=>(o[i]=v,o), {})

[docs]

or more verbose

var trAr2Obj = function (arr) {return arr.reduce((o,v,i)=>(o[i]=v,o), {});}

or

var transposeAr2Obj = arr=>arr.reduce((o,v,i)=>(o[i]=v,o), {})

shortest one with vanilla JS

JSON.stringify([["a", "X"], ["b", "Y"]].reduce((o,v,i)=>{return o[i]=v,o}, {}))
=> "{"0":["a","X"],"1":["b","Y"]}"

some more complex example

[["a", "X"], ["b", "Y"]].reduce((o,v,i)=>{return o[v[0]]=v.slice(1)[0],o}, {})
=> Object {a: "X", b: "Y"}

even shorter (by using function(e) {console.log(e); return e;} === (e)=>(console.log(e),e))

 nodejs
> [[1, 2, 3], [3,4,5]].reduce((o,v,i)=>(o[v[0]]=v.slice(1),o), {})
{ '1': [ 2, 3 ], '3': [ 4, 5 ] }

[/docs]

Solution 18

As of Lodash 3.0.0 you can use _.toPlainObject

var obj = _.toPlainObject(['a', 'b', 'c']);
console.log(obj);
<script src="https://cdn.jsdelivr.net/lodash/4.16.4/lodash.min.js"></script>

Solution 19

If you can use Map or Object.assign, it's very easy.

Create an array:

const languages = ['css', 'javascript', 'php', 'html'];

The below creates an object with index as keys:

Object.assign({}, languages)

Replicate the same as above with Maps

Converts to an index based object {0 : 'css'} etc...

const indexMap = new Map(languages.map((name, i) => [i, name] ));
indexMap.get(1) // javascript

Convert to an value based object {css : 'css is great'} etc...

const valueMap = new Map(languages.map(name => [name, `${name} is great!`] ));
valueMap.get('css') // css is great

Solution 20

If someone is searching for a Typescript method, i wrote this:

const arrayToObject = <T extends Record<K, any>, K extends keyof any>(
  array: T[] = [],
  getKey: (item: T) => K,
) =>
  array.reduce((obj, cur) => {
    const key = getKey(cur)
    return ({...obj, [key]: cur})
  }, {} as Record<K, T>)

It will:

  1. enforce first param to be array of objects
  2. help to select the key
  3. enforce the key to be an key of all array items

Example:

// from:
const array = [
    { sid: 123, name: 'aaa', extra: 1 },
    { sid: 456, name: 'bbb' },
    { sid: 789, name: 'ccc' }
];
// to:
{
  '123': { sid: 123, name: 'aaa' },
  '456': { sid: 456, name: 'bbb' },
  '789': { sid: 789, name: 'ccc' }
}

usage:

const obj = arrayToObject(array, item => item.sid) // ok
const obj = arrayToObject(array, item => item.extra) // error

Here's a demo.

Solution 21

Here's a recursive function I just wrote. It's simple and works well.

// Convert array to object
var convArrToObj = function(array){
    var thisEleObj = new Object();
    if(typeof array == "object"){
        for(var i in array){
            var thisEle = convArrToObj(array[i]);
            thisEleObj[i] = thisEle;
        }
    }else {
        thisEleObj = array;
    }
    return thisEleObj;
}

Here's an example (jsFiddle):

var array = new Array();
array.a = 123;
array.b = 234;
array.c = 345;
var array2 = new Array();
array2.a = 321;
array2.b = 432;
array2.c = 543;
var array3 = new Array();
array3.a = 132;
array3.b = 243;
array3.c = 354;
var array4 = new Array();
array4.a = 312;
array4.b = 423;
array4.c = 534;
var array5 = new Array();
array5.a = 112;
array5.b = 223;
array5.c = 334;

array.d = array2;
array4.d = array5;
array3.d = array4;
array.e = array3;


console.log(array);

// Convert array to object
var convArrToObj = function(array){
    var thisEleObj = new Object();
    if(typeof array == "object"){
        for(var i in array){
            var thisEle = convArrToObj(array[i]);
            thisEleObj[i] = thisEle;
        }
    }else {
        thisEleObj = array;
    }
    return thisEleObj;
}
console.log(convArrToObj(array));

Results:

Solution 22

I would do this simply with Array.of(). Array of has the ability to use it's context as a constructor.

NOTE 2 The of function is an intentionally generic factory method; it does not require that its this value be the Array constructor. Therefore it can be transferred to or inherited by other constructors that may be called with a single numeric argument.

So we may bind Array.of() to a function and generate an array like object.

function dummy(){};
var thingy = Array.of.apply(dummy,[1,2,3,4]);
console.log(thingy);

By utilizing Array.of() one can even do array sub-classing.

Solution 23

let i = 0;
let myArray = ["first", "second", "third", "fourth"];

const arrayToObject = (arr) =>
    Object.assign({}, ...arr.map(item => ({[i++]: item})));

console.log(arrayToObject(myArray));

Or use

myArray = ["first", "second", "third", "fourth"]
console.log({...myArray})

Solution 24

ES5 - Solution:

Using Array prototype function 'push' and 'apply' you can populate the object with the array elements.

var arr = ['a','b','c'];
var obj = new Object();
Array.prototype.push.apply(obj, arr);
console.log(obj);    // { '0': 'a', '1': 'b', '2': 'c', length: 3 }
console.log(obj[2]); // c

Solution 25

Try using reflect to copy from array item to object.

var arr =['aa:23','bb:44','cc:55']
    var obj ={}
    arr.forEach(e => {
        var ee = e.split(':')
        Reflect.set(obj,ee[0],ee[1])
    });
    console.log(obj) // { aa: '23', bb: '44', cc: '55' }

Solution 26

You could use a function like this:

var toObject = function(array) {
    var o = {};
    for (var property in array) {
        if (String(property >>> 0) == property && property >>> 0 != 0xffffffff) {
            o[i] = array[i];
        }
    }
    return o;
};

This one should handle sparse arrays more efficiently.

Solution 27

It's not directly relevant but I came here searching for a one liner for merging nested objects such as

const nodes = {
    node1: {
        interfaces: {if1: {}, if2: {}}
    },
    node2: {
        interfaces: {if3: {}, if4: {}}
    },
    node3: {
        interfaces: {if5: {}, if6: {}}
    },
}

The solution is to use a combination of reduce and object spread:

const allInterfaces = nodes => Object.keys(nodes).reduce((res, key) => ({...res, ...nodes[key].interfaces}), {})

Solution 28

Simplest way to do this is the following:

const arr = ['a','b','c'];
let obj = {}

function ConvertArr(arr) { 
 if (typeof(arr) === 'array') {
 Object.assign(obj, arr);
}

This way it only runs if it is an array, however, you can run this with let global object variable or without, that's up to you, if you run without let, just do Object.assign({}, arr).

Solution 29

Use the javascript lodash library. There is a simple method _.mapKeys(object, [iteratee=_.identity]) that can do the conversion.

Solution 30

Here's a solution in coffeescript

arrayToObj = (arr) ->
  obj = {}
  for v,i in arr
    obj[i] = v if v?
  obj