When using the Javascript built in Map, how do you use .map() to iterate over the keys?
I know the for...of can be used as shown below:
const map = new Map();
map.set(0, 'Zero');
map.set(1, 'One');
map.set(2, 'Two');
for (let key of map.keys()) {
console.log(key);
}
But this code will fail:
map.keys().map(key => {
console.log(key);
});
Map.keys() returns an iterator you can spread the iterator using spread syntax
const map = new Map();
map.set(0, 'Zero');
map.set(1, 'One');
map.set(2, 'Two');
[...map.keys()].forEach(key => {
console.log(key);
})
for..of because it conforms to the iterator protocol already.
It's Array.prototype.map actually, it's defined for arrays, so use Array.from to convert the keys to an array and then use map:
const map = new Map();
map.set(0, 'Zero');
map.set(1, 'One');
map.set(2, 'Two');
console.log(...Array.from(map.keys()).map(key => {
return key ** 2; // square the keys
}));You can use Array.from():
const map = new Map();
map.set(0, 'Zero');
map.set(1, 'One');
map.set(2, 'Two');
Array.from(map.keys()).map(key => {
console.log(key);
});Hopefully that helps!
You might be better off using Array.from mapping function directly to avoid creating a temporary array:
Array.from(map.keys(), k => console.log(k))
Another, more verbose, but helpful option would be to redefine array iteration methods on the iterator prototype, thus making them automatically available for all iterators:
// http://www.ecma-international.org/ecma-262/7.0/#sec-%iteratorprototype%-object
const IteratorPrototype = Object.getPrototypeOf(Object.getPrototypeOf([][Symbol.iterator]()));
Object.defineProperties(IteratorPrototype, {
forEach: {
value: function (fn) {
let n = 0;
for (let x of this) {
fn(x, n++, this);
}
},
enumerable: false
},
map: {
value: function (fn) {
let n = 0, a = [];
for (let x of this) {
a.push(fn(x, n++, this));
}
return a;
},
enumerable: false
},
reduce: {
value: function (fn, init) {
let n = 0;
if (arguments.length === 1) {
init = this.next().value;
}
for (let x of this) {
init = fn(init, x, n++, this);
}
return init;
},
enumerable: false
},
});
/////
const map = new Map();
map.set('a', 'Zero');
map.set('b', 'One');
map.set('c', 'Two');
map.keys().map(console.log)
console.log(map.values().reduce((o, k) => o + '/' + k));
function* it() {
yield 'x';
yield 'y';
yield 'z';
}
it().map(x => console.log(x))map.forEach((_,key) => console.log(key));
const map = new Map([[0, "Zero"], [1, "One"], [2, "Two"]]);
const arr = Array.from(map, (entry) => ({ key: entry[0], value: entry[1] }));
console.log(arr);
