First item from a Map on JavaScript ES2015

Question

I have a Map like this:

const m = new Map(); m.set('key1', {}) . m.set('keyN' {}) 

the Mapcan have 1 or many items. Can I get the first item by index, without m.get('key1') and without a iterator loop?

like: m.get()[0]

Answer 1

Use the Map.prototype.entries function, like this

const m = new Map();  m.set('key1', {})  m.set('keyN', {})    console.log(m.entries().next().value); // [ 'key1', {} ]

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If you want to get the first key, then use Map.prototype.keys, like this

console.log(m.keys().next().value); // key1 

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Similarly if you want to get the first value, then you can use Map.prototype.values, like this

console.log(m.values().next().value); // {} 

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The reason why we have to call next() on the returned values is that, all those functions return iterators. Read more about the iteration protocol here.

Answer 2

For the specific example you are wondering about, destructuring would be perfect.

let m = new Map(); m.set('key1', {}); m.set('key2', {});  let [[, obj]] = m; 

e.g.

let [pair] = m; let [key, obj] = pair; 

is one option to destructure and then grab the value, but the easier option would be

let [obj] = m.values();