Convert JavaScript array of 2 element arrays into object key value pairs

Question

What is the fastest algorithm for getting from something like this:

var array = [ [1,'a'], [2,'b'], [3,'c'] ];

to something like this:

Object { 1: "a", 2: "b", 3: "c" }

so far this is what i've come up with:

function objectify(array) {
    var object = {};
    array.forEach(function(element) {
        object[element[0]] = element[1];
    });
    return object;
}

which works fine, but it seems kind of clumsy. Is there a better way? Would something like reduce() work and would that be any faster?

Penny Liu · Accepted Answer · 2023-03-16 05:47:20Z

35

You can use Object.fromEntries to convert an Array to an Object:

var array = [
  [1, 'a'],
  [2, 'b'],
  [3, 'c']
];
var object = Object.fromEntries(array);
console.log(object);

edited Mar 16, 2023 at 5:47

answered Jun 19, 2019 at 7:19

Penny Liu

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2 Comments

tel Over a year ago

This is the "correct" answer, and much nicer than the reduce approach. Only use reduce if you need to target very old browsers (eg IE)

Arun Ramachandran Over a year ago

What an elegant way of getting things done. This code snippet does the job without any further complications.

Alnitak · Accepted Answer · 2014-10-19 19:57:34Z

22

You could indeed use Array.prototype.reduce:

function objectify(array) {
    return array.reduce(function(p, c) {
         p[c[0]] = c[1];
         return p;
    }, {});
}

where p is the result of the previous iteration, initially {}, and c is the current element of the array.

It's unlikely to be any faster than array.forEach, but it is IMHO cleaner. I don't believe there's any simpler implementation than this.

NB: a function to do exactly this already exists in the Underscore library: _.object(array)

edited Oct 19, 2014 at 19:57

answered Oct 19, 2014 at 19:51

Alnitak

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Comments

Phrogz · Accepted Answer · 2017-07-01 02:18:04Z

9

Terse version using modern syntax:

let objectify = a => a.reduce( (o,[k,v]) => (o[k]=v,o), {} );

I use this technique as part of a terse query string parser:

// Converts "?foo=bar&j=1&go" into { foo:'bar', j:'1', go:true }
function parseQueryString(qs) {
    var q = decodeURIComponent;
    return qs.replace(/^\?/,'').split('&').map(s => s.split('='))
             .reduce((o,[k,v]) => (o[q(k)] = v?q(v):true, o), {});
}

answered Jul 1, 2017 at 2:18

Phrogz

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2 Comments

KyleFarris Over a year ago

This works perfectly for my use case. I consider myself pretty proficient in ESXXXX but I can't quite figure out how that lambda is working. Specifically the (o[k]=v,o) part. Any insights? Also, thanks for sharing this gem. :-)

Alnitak Over a year ago

@KyleFarris it's using the comma operator to perform the assignment into the map, and then return o itself.

VLAZ · Accepted Answer · 2022-06-01 15:03:09Z

7

Lodash has a _.fromPairs method that does exactly that.

From the documentation:

_.fromPairs([['a', 1], ['b', 2]]);
// => { 'a': 1, 'b': 2 }

edited Jun 1, 2022 at 15:03

VLAZ

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answered Jun 28, 2017 at 8:57

Uri Klar

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Comments

thefourtheye · Accepted Answer · 2014-10-19 19:51:26Z

2

You can wrap the entire thing within Array.prototype.reduce, like this

function objectify(array) {
    return array.reduce(function(result, currentArray) {
        result[currentArray[0]] = currentArray[1];
        return result;
    }, {});
}

console.log(objectify([ [1, 'a'], [2, 'b'], [3, 'c'] ]));
# { '1': 'a', '2': 'b', '3': 'c' }

We are just accumulating the key-value pairs in the result object and finally the result of reduce will be the result object and we are returning it as the actual result.

answered Oct 19, 2014 at 19:51

thefourtheye

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