Convert Array to Object

What is the best way to convert:

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

to:

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

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" }

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.

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.

If you're using jquery:

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

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

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

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' }

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}), {})

Surprised not to see -

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

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

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)

I ended up using object spread operator, since it is part of the ECMAScript 2015 (ES6) standard.

const array = ['a', 'b', 'c'];
console.log({...array});
// it outputs {0:'a', 1:'b', 2:'c'}

Made the following fiddle as an example.

For ES2016, spread operator for objects. Note: This is after ES6 and so transpiler will need to be adjusted.

const arr = ['a', 'b', 'c'];
const obj = {...arr}; // -> {0: "a", 1: "b", 2: "c"} 

Object.assign({}, ['one', 'two']); // {0: 'one', 1: 'two'}

Easy way in modern JavaScript is to use Object.assign() that does nothing but copying key:value from one object to another. In our case, Array donates properties to new {}.

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;

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);

Five years later, there's a good way :)

Object.assign was introduced in ECMAScript 2015.

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

you can use spread operator

x = [1,2,3,10]
{...x} // {0:1, 1:2, 2:3, 3:10}

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]})))

The simplest way is to use a spread operator.

var arr = ["One", "Two", 3];
var obj = {...arr};
console.log(obj);

A quick and dirty one:

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

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

Quick and dirty #2:

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

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

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;
};

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"}

.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]

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>

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

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.

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:

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.

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})

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

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

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' }

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.

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}), {})

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).

Here's a solution in coffeescript

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

If you are using angularjs you can use angular.extend, the same effect with $.extend of jquery.

var newObj = {};
angular.extend(newObj, ['a','b','c']);

Below method would convert array to object with particular given key.

    /**
     * Converts array to object
     * @param  {Array} array
     * @param  {string} key (optional)
     */
    Array.prototype.ArrayToObject = function(key) {
       const array = this;
	   const obj = {};
      
       array.forEach((element, index) => {
           if(!key) {
              obj[index] = element;
           } else if ((element && typeof element == 'object' && element[key])) {
              obj[element[key]] = element;
           }
       });
    return obj;
    }

For Ex -

[{name: 'test'}, {name: 'test1'}].ArrayToObject('name');

would give

{test: {name: 'test'}, test1: {name: 'test1'}}

and incase key is not provided as param to the method

i.e. [{name: 'test'}, {name: 'test1'}].ArrayToObject();

would give

{0: {name: 'test'}, 1: {name: 'test1'}}

maybe if you want array value to be your object key too

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

typescript solutioin:

export const toMap = (errors: ResponseError[]) => {
  const errorMap: Record<string, string> = {};
  errors.forEach(({ field, message }) => {
    errorMap[field] = message;
  });
  return errorMap;
};

export type FieldError = {
  field: string;
  message: string;
};

I have faced this issue multiple times and decided to write a function that is as generic as possible. Have a look and feel free to modify anything

function typeOf(obj) {
	if ( typeof(obj) == 'object' ) {
		if (obj.length)
			return 'array';
		else
			return 'object';
	} else
	return typeof(obj);
}

function objToArray(obj, ignoreKeys) {
	var arr = [];
	if (typeOf(obj) == 'object') {
		for (var key in obj) {
			if (typeOf(obj[key]) == 'object') {
				if (ignoreKeys)
					arr.push(objToArray(obj[key],ignoreKeys));
				else
					arr.push([key,objToArray(obj[key])]);
			}
			else
				arr.push(obj[key]);
		}
	}else if (typeOf(obj) == 'array') {
		for (var key=0;key<obj.length;key++) {
			if (typeOf(obj[key]) == 'object')
				arr.push(objToArray(obj[key]));
			else
				arr.push(obj[key]);
		}
	}
	return arr;
}

I would use underscore for this, but if that isn't available then I would drop down to using reduce with an initial value of empty object {}

>>> ['a', 'b', 'c'].reduce(function(p, c, i) {p[i] = c; return p}, {})
Object { 0="a", 1="b", 2="c"}

reduce should be widely available in most browsers today, see MDN

If you like oneliners, and IE8 is not a problem anymore (as it should be)

['a','b','c'].reduce((m,e,i) => Object.assign(m, {[i]: e}), {});

Go ahead and try it on the browser console

It coult be more verbose like this:

['a','b','c'].reduce(function(memo,elm,idx) {
    return Object.assign(memo, {[idx]: elm});
}, {});

But still rules out IE8. If IE8 is a must, then you can use lodash/underscore like that:

_.reduce(['a','b','c'], function(memo,elm,idx) {
    return Object.assign(memo, {[idx]: elm});
}, {})

It is easy to use javascript reduce:

["a", "b", "c", "d"].reduce(function(previousValue, currentValue, index) { 
   previousValue[index] = currentValue; 
   return previousValue;
}, 
{}
);

You can take a look at Array.prototype.reduce(), https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

initial array and will convert into an Object with keys which will be the unique element of an array and the keys value will be how many times the perticular keys will be repeating

var jsTechs = ['angular', 'react', 'ember', 'vanilaJS', 'ember', 'angular', 'react', 'ember', 'vanilaJS', 'angular', 'react', 'ember', 'vanilaJS', 'ember', 'angular', 'react', 'ember', 'vanilaJS', 'ember', 'angular', 'react', 'ember', 'vanilaJS', 'ember', 'react', 'react', 'vanilaJS', 'react', 'vanilaJS', 'vanilaJS']

var initialValue = {
  java : 4
}

var reducerFunc = function reducerFunc (initObj, jsLib) {
  if (!initObj[jsLib]) {
    initObj[jsLib] = 1
  } else {
    initObj[jsLib] += 1
  }
  return initObj
}
var finalResult = jsTechs.reduce(reducerFunc, initialValue)
console.log(finalResult)

My version array to json in JS. Jusy copy/paste and use it. Isn't this awesome? I love this type of functions I found on StackOverflow.

function array2json(arr) {
    var parts = [];
    var is_list = (Object.prototype.toString.apply(arr) === '[object Array]');

    for(var key in arr) {
	    var value = arr[key];
	    if(typeof value == "object") { //Custom handling for arrays
		    if(is_list) parts.push(array2json(value)); /* :RECURSION: */
		    else parts[key] = array2json(value); /* :RECURSION: */
	    } else {
		    var str = "";
		    if(!is_list) str = '"' + key + '":';

		    //Custom handling for multiple data types
		    if(typeof value == "number") str += value; //Numbers
		    else if(value === false) str += 'false'; //The booleans
		    else if(value === true) str += 'true';
		    else str += '"' + value + '"'; //All other things
		    // :TODO: Is there any more datatype we should be in the lookout for? (Functions?)

		    parts.push(str);
	    }
    }
    var json = parts.join(",");

    if(is_list) return '[' + json + ']';//Return numerical JSON
    return '{' + json + '}';//Return associative JSON
}

var finalResult = ['a','b','c'].map((item , index) => ({[index] : item}));
console.log(finalResult)

A more OO approach:

Array.prototype.toObject = function() {
 var Obj={};
    
 for(var i in this) {
  if(typeof this[i] != "function") {
   //Logic here
   Obj[i]=this[i];
  }
 }

 return Obj;
}