JavaScript Map with objects for keys: duplicate entries-CodePudding

According to the mdn webdocs on JavaScript's Map:

A Map's keys can be any value (including functions, objects, or any primitive).

However:

const map = new Map();
console.log(map.size); // 0

map.set({someProperty: 0}, 0);
console.log(map.size); // 1

map.set({someProperty: 0}, 0);
console.log(map.size); // 2 <-- what? shouldn't this be 1 still?

The documentation indicates that it's fine to use an object as a key. So how can I insert an object with the same key into the Map?

CodePudding user response：

The two objects have different references.

If you use the same object both times, you only have one entry:

const map = new Map();
const key = { someProperty: 0 };
map.set(key, 0);
map.set(key, 0);
console.log(map.size);

CodePudding user response：

Every object is unique (even if its keys and values are identical to those of another one), so when you use different objects as keys, each will occupy a different entry in the Map.

See more at Equality comparisons and sameness on MDN.

const map = new Map();
console.log(map.size); // 0

const a = {someProperty: 0};
const b = {someProperty: 0};

console.log(a === b); // false

map.set(a, 0);
console.log(map.size); // 1

map.set(b, 0);
console.log(map.size); // 2

const c = a;

console.log(a === c); // true

map.set(c, 1);
console.log(map.size); // 2

console.log(map.get(a)); // 1 (not 0)