Converting an Array to an Object

Question

I am trying to write a function which takes in an array of arrays, and returns an object with each pair of elements in the array as a key-value pair. I have searched stack overflow and came up with the following code. However, my code below only returns the first array { make: 'Ford' }. My code does not return the rest of the arrays. Any suggestions on why my function does not return the rest of the array of arrays?

var array = [
  ['make', 'Ford'],
  ['model', 'Mustang'],
  ['year', 1964]
];

function fromListToObject() {

  for (var i = 0; i < array.length - 1; ++i) {
    var object = {};
    //creates an object as a variable
    var newArray = array[i];
    //creates a variable for the array within the array
    object[i] = newArray[1];
    //sets the object of the element to position 1 of the array within the array
    object[newArray[0]] = object[i];
    //sets the key of the element to position 0 of the array within the array
    delete object[i];
    // removes the previous key of 0
    return object;
  }
}

console.log(fromListToObject(array));

Try walking through your code with the debugger. Step by step. You might add `object` as a "watch" to see how its value changes. — , Mar 23 '17 at 05:33
Possible duplicate of [Convert JavaScript array of 2 element arrays into object key value pairs](http://stackoverflow.com/questions/26454655/convert-javascript-array-of-2-element-arrays-into-object-key-value-pairs) — Rajesh, Mar 23 '17 at 05:34
Thank you all for your solutions. I tried to declare the object outside of the loop but it didn't seem to work..The function still only returned the first array of arrays. — user1137894, Mar 23 '17 at 05:45

Geethu Jose · Answer 1 · 2017-03-23T05:53:06.203

1

Try this

You have declared obj inside the loop. During iteration

object = Object {make: "Ford"}, i = 0

After the first iteration your function

console.log(fromListToObject(array));

is called. This is because you call the return inside the loop, this should be done outside the loop. Rewrite your code as

var array = [
    ['make', 'Ford'],
    ['model', 'Mustang'],
    ['year', 1964]
  ],
  i, j = array.length,
  object = {}

function fromListToObject() {

  for (i = 0; i < j; i++) {
    var newArray = array[i];
    object[i] = newArray[1];
    object[newArray[0]] = object[i];
    delete object[i];
  }
  return object;
}

console.log(fromListToObject(array));

var array = [['make', 'Ford'], ['model', 'Mustang'], ['year', 1964]];

var obj = {};
array.forEach(function(data){
    obj[data[0]] = data[1]
});
console.log(obj);

edited Mar 23 '17 at 05:53

answered Mar 23 '17 at 05:31

Geethu Jose

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That works just fine, but what is wrong with the OP's code? – Mar 23 '17 at 05:34
In OP's code, the result obtained is just { "make": "Ford" } – Geethu Jose Mar 23 '17 at 05:38
1

torazaburo mean, you should explain **why** OP's code was failing and why your code gives proper output. Remember, you are answering for readers and not just OP. – Rajesh Mar 23 '17 at 05:39
Thanks Geethu! I was curious to understand why my code didn't work but your code works as well!! – user1137894 Mar 23 '17 at 05:43

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

Your code has several issues.

The target object is reset to {} in each iteration. So you should move var object = {} out of your loop.
You return from inside your loop, which immediately ends it: that should happen after the loop.

You iterate one time too few. Remove the - 1:

   for (var i = 0; i < array.length; ++i) {

There is an assignment to a numerical property, which you delete 2 lines later: that is a costly operation, which you should avoid. Just do the assignment in one operation:
```
   object[newArray[0]] = newArray[1];
```

Here is everything corrected:

var array = [['make', 'Ford'], ['model', 'Mustang'], ['year', 1964]];

function fromListToObject() {
  var object = {}; // out of the loop
  for (var i = 0; i < array.length; ++i) { // iterate to last
    var newArray = array[i];
    object[newArray[0]] = newArray[1];
  }
  return object; // out of the loop
}

var obj = fromListToObject(array);

console.log(obj);

ES6 Version

If you can use ES6 syntax, then consider this solution:

function fromListToObject(array) {
    return Object.assign(...array.map( ([key,val]) => ({[key]: val}),{}));
}

const array = [['make', 'Ford'], ['model', 'Mustang'], ['year', 1964]];
const obj = fromListToObject(array);
console.log(obj);

Using Map

You might also be interested in the ES6 Map object, which works similarly to an object, and can be constructed from your array in the most simple way:

const mp = new Map(array);

You can then access the key values as follows:

mp.get('make') === 'Ford';

And iterate:

mp.forEach( function (val, key) {
    console.log(key, val);
});

Thanks trincot. I was curious to understand why my code didn't work but your code works as well!! — user1137894, Mar 23 '17 at 05:42
I added a list of issues in your code and a version of it that has it corrected. — trincot, Mar 23 '17 at 05:52

score 0 · Answer 3 · answered Mar 23 '17 at 06:41

While: Not Counting!

object = {};
while( array[0] )object[ array[0][0] ] = array[0][1], array.shift();

will result in:

object;
>> Object { make: "Ford", model: "Mustang", year: 1964 }

array;
>> Array [ ]

This action will actually dispatch array members for patching the object! The array content will be physically dislocated to the newly declared object.

Converting an Array to an Object

3 Answers3

ES6 Version

Using Map

Linked

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