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. — user663031
– user663031, Commented Mar 23, 2017 at 5:33
Possible duplicate of Convert JavaScript array of 2 element arrays into object key value pairs — Rajesh Dixit
– Rajesh Dixit, Commented Mar 23, 2017 at 5:34
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
– user1137894, Commented Mar 23, 2017 at 5:45

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

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);
});

edited Jun 20, 2020 at 9:12

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answered Mar 23, 2017 at 5:33

trincot

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2 Comments

user1137894 Over a year ago

Thanks trincot. I was curious to understand why my code didn't work but your code works as well!!

trincot Over a year ago

I added a list of issues in your code and a version of it that has it corrected.

Geethu Jose · Accepted Answer · 2017-03-23 05:53:06Z

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, 2017 at 5:53

answered Mar 23, 2017 at 5:31

Geethu Jose

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4 Comments

user663031 Over a year ago

That works just fine, but what is wrong with the OP's code?

Geethu Jose Over a year ago

In OP's code, the result obtained is just { "make": "Ford" }

Rajesh Dixit Over a year ago

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.

user1137894 Over a year ago

Thanks Geethu! I was curious to understand why my code didn't work but your code works as well!!

Bekim Bacaj · Accepted Answer · 2017-03-23 06:41:40Z

0

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.

answered Mar 23, 2017 at 6:41

Bekim Bacaj

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