I am working with a Map structured as follows:
const m = new Map();
m.set('key1', {})
.
m.set('keyN' {})
The Map may contain one or multiple items. I am wondering if there is a way to retrieve the first item by index, without using m.get('key1') and avoiding an iterator loop?
For instance, can I do something like: m.get()[0]
One way to retrieve entries from a Map in JavaScript is by using the Map.prototype.entries method as shown below:
const m = new Map();
m.set('key1', {})
m.set('keyN', {})
console.log(m.entries().next().value); // [ 'key1', {} ]If you specifically need to access the first key in the Map, you can achieve that by utilizing the Map.prototype.keys method like so:
console.log(m.keys().next().value); // key1
For retrieving the first value in the Map, you can use the Map.prototype.values method, demonstrated here:
console.log(m.values().next().value); // {}
The necessity of calling next() on the outputs of these methods stems from their return of iterators. To delve deeper into the iteration protocol and how it works, refer to this guide on iterators and generators. More information on iteration protocols can be found here.
In the particular scenario you're inquiring about, utilizing destructuring would be ideal.
const myMap = new Map();
myMap.set('key1', {});
myMap.set('key2', {});
let [[, object]] = myMap;
For example:
let [pair] = myMap;
let [key, object] = pair;
One approach to destructure and retrieve the value is shown above, however a simpler option would be
let [object] = myMap.values();
To achieve this, you can utilize the spread feature available in ES6 and later versions of JavaScript. Begin by declaring a new Map variable and adding two values to it. Then, you can use ... to convert the map into an array, or alternatively, you can employ Array.from. To access the first element, simply use [0] on the resulting array.
const myMap = new Map();
myMap.set('key1', 1);
myMap.set('key2', 2);
console.log([...myMap][0]); // ['key1', 1] 👍🏼You can also achieve this quickly by using destructuring for JavaScript arrays. By doing so, the [k, v] array will represent the first item in the map.
const [[k, v]] = myMap;
console.log(k, v); // 'key1', 1
Indeed, this method works well for both Set and Map. You can easily convert them to an Array and then access any element by its index. Here's an example:
const s = new Set();
s.add('item1');
s.add('item2');
console.log(Array.from(s)[0]); // ['item1']When working with iterable objects, you can utilize the iterator object[Symbol.iterator]().
In this scenario, it will refer to the entries() method as described in the MDN page mentioned above:
The map iterator function defaults to the entries() function.
const m = new Map();
m.set('key1', {})
m.set('keyN', {})
console.log(m[Symbol.iterator]().next().value); // [ 'key1', {} ]Check out a benchmark of all solutions here:
The version using entries() comes out on top, closely followed by the iterator version. This makes sense since [Symbol.iterator]() essentially calls entries().
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