Convert an array of objects with a unique id property to a Map

I have an array of objects, where each object has a unique member called id. How do I create a Map where the id if the Map's key?

Answers:

Answer

You want to reduce your array into a map:

const arr = [{id:1},{id:2},{id:2}];

const map = arr.reduce((acc, item) => acc.set(item.id, item), new Map());

console.log(map.get(1));

Here is a JSPref against using map and forEach.

In Chrome v53 reduce is fastest, then forEach with map being the slowest.

Answer

You can use Array.prototype.map() to map the array elements to [element.id, element] pairs and then pass the resulting array to the Map constructor.

const arr = [{id: 1, a: true, b: false}, {id: 2, a: false, b: true}]

const map = new Map(arr.map(element => [element.id, element]))

// Check if map looks OK
for (const [key, value] of map) {
  console.log(key, value)
}

Answer

You could map a new array in the needed format for the Map.

var array = [{ id: 1, value: 'one' }, { id: 2, value: 'two' }, { id: 3, value: 'three' }, { id: 4, value: 'four' }, { id: 5, value: 'five' }],
    map = new Map(array.map(a => [a.id, a]));

console.log([...map]);
.as-console-wrapper { max-height: 100% !important; top: 0; }

Or iterate and add the new item to a certain key

var array = [{ id: 1, value: 'one' }, { id: 2, value: 'two' }, { id: 3, value: 'three' }, { id: 4, value: 'four' }, { id: 5, value: 'five' }],
    map = new Map();

array.forEach(a => map.set(a.id, a));
console.log([...map]);
.as-console-wrapper { max-height: 100% !important; top: 0; }

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