2

Map is a new object in ECMA6, if assign multiple value to the same key, it will overwrite all previous values. For example,

'use strict';

var m = new Map();

m.set(['aaron', 100]);
m.set(['aaron', 100], 1);
m.set(['aaron', 100], 10);
m.set(['aaron', 100], 100);
m.set(['aaron', 100], 1000);
m.set(['aaron', 100], 10000);

console.log(m.get(['aaron', 100]));

It will show a weird output(undefined), why? Many thanks.

mplungjan
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Aaron Chen
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2 Answers2

4

The Map uses as the key the reference to the array, and not the contents of the array.

This simple comparison shows that arrays with the same content, are not the same array (have different reference):

const a = ['aaron', 100];
const b = ['aaron', 100];

console.log(a === b);

It works if you set and get the same reference:

const arr = ['aaron', 100];
const m = new Map();
m.set(arr);
m.set(arr, 1);
m.set(['aaron', 100], 10);
m.set(['aaron', 100], 100);
m.set(['aaron', 100], 1000);
m.set(['aaron', 100], 10000);

console.log(m.get(arr));
Ori Drori
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1

In JavaScript, Array, Object or Function are Reference types, when you compare them what you actually do is comparing the references to their locations in memory, not their values. As they both allocated to difference locations, the result will return "false".

What you should do is convert them into a primitive type. JSON string for example:

let a = {1: 'foo'},
    b = {1: 'foo'};
    
console.log(a === b);
console.log(JSON.stringify(a) == JSON.stringify(b));
Thien
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