Convert Array to Object

Question

What is the best way to convert:

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

to:

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

Answer 1

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.

Answer 2

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)

Answer 3

If you're using jquery:

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

Answer 4

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

Answer 5

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:

Answer 6

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.