Convert array to object in JavaScript

Question

I'd like to convert an array into an object using one of the properties as key.

For instance:

var stations = [ { name: 'A Coruna', ds100: 'OECSC' }, ... ];

I'd like to get stationByDs100:

{ 'OECSC': { name: 'A Coruna', ds100: 'OECSC' }, ... }

At the moment I do it as follows:

var stationByDs100 = {};
stations.forEach(station => stationByDs100[station.ds100] = station);

I'm looking for a better way to accomplish this - is it possible with a one-liner without explicit variable declaration?

For instance in Java 8 this could have accomplished with streams and collectors in one line like:

Map<String, Station> stationByDs100 = stations.stream()
    .collect(toMap(Station::getDs100, identity()));

So I was thinking maybe there's a similar way in JS.

I'm using Node.js so OK to use the latest JS/ES features Node.js supports.

I've browsed through a rougly dozen existing answers but they mostly use older JS/ES versions and suggest even longer solutions.

@SpencerWieczorek One-liner, without explicit variable declaration, from said criteria it is better. — lexicore
– lexicore, Commented Jan 29, 2017 at 10:17
Is there a specific purpose for wanting the code to be a one-liner? Usually one-line code, especially involving functional-style array methods, are slower than the equivalent code using for/while loops and more than one line of code — Zorgatone
– Zorgatone, Commented Jan 29, 2017 at 10:19
@Zorgatone No technical reason. I don't care about performance in this case and wanted to learn more about functional style in JS as I'm quite weak in that. — lexicore
– lexicore, Commented Jan 29, 2017 at 10:21

Nina Scholz · Accepted Answer · 2017-01-29 10:14:35Z

9

You could use Object.assign with computed property names.

var stations = [ { name: 'A Coruna', ds100: 'OECSC' }],
    object = stations.reduce((o, a) => Object.assign(o, { [a.ds100]: a }), {});

console.log(object);

answered Jan 29, 2017 at 10:14

Nina Scholz

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3 Comments

NonPolynomial Over a year ago

I would prefer Object.assign({}, o, { [a.ds100]: a }) instead of Object.assign(o, { [a.ds100]: a }) to prevent side effects. just a personally preference.

Nina Scholz Over a year ago

the difference is, i return the same object, while NonPolynomial suggest to use a new object all time.

lexicore Over a year ago

@NinaScholz Yes, I got it, thank you. I think it's not really necessary in this case as accumulator object is only used in reduction.

trincot · Accepted Answer · 2025-01-02 13:10:47Z

0

Object.fromEntries can help with this:

const stations = [{ name: 'Berlin Südkreuz', ds100: 'BPAF' }, { name: 'Berg', ds100: 'KBEG' }];
const object = Object.fromEntries(stations.map(o => [o.ds100, o]));
console.log(object);

answered Jan 2 at 13:10

trincot

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