JavaScript 映射对象不保存对象键？

Question

So I'm trying to add an entry to a map object with the key being an object but it doesn't seem to work.

let myMap = new Map();
myMap.set({type : 'sum', val : 10},1);
console.log(myMap.get({type:'sum',val:10})); <---- this returns undefined

why is it returning undefined instead of 1?

Answer 1

The reason is that objects are references to values in JavaScript. So two objects, even with the identical value are evaluated as different, since its stored as two different values. If you use the same, you get the result you expect:

let myMap = new Map();
const key = {type : 'sum', val : 10}
myMap.set(key, 1);
console.log(myMap.get(key)); // 1

Answer 2

javascriot uses SameValueZero algorithm to campare key when you set and get from this structure. SameValueZero is similar to strict equality (===). since in get method you are creating new object and pass it to this method as key this key is not strictly equal to the object you use when you set so you get null.

Answer 3

It should accept key value pair only, so your are providing object in place of key.

 myMap.set({type : 'sum', val : 10},1);
 myMap.set( {key:1, val: {type : 'sum', val : 10}} );

Try using a key and adding the object { type:'',val:''} to it as its value. Try wrapping '1' into the object itself

As you will need a key to access it.

Ref: [ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map][1]

It will return myMap.size

Answer 4

Even if your two objects used as a key looks equals (because they have the same attributes), they aren't (according to === which is more or less the operation used - see here for more details ).

It will work if you do the following:

let myMap = new Map();
var obj = {type : 'sum', val : 10};
myMap.set(obj,1);
console.log(myMap.get(obj));

Answer 5

Map is a key value pair where you store a value with it's key. Like:

let map = new Map();
         // key     // value
map.set('myKey','just a text');

then to access that value, you need the key in this case which is 'myKey'

map.get('myKey'); // will return just a text

if you wish to convert your object to a map, follow this article :

let obj = {
  name: "John",
  age: 30
};

let map = new Map(Object.entries(obj));

alert( map.get('name') ); // John

Answer 6

You use 2 different objects that just happen to have the same properties. They are not equal, though. To make it work, you would have to use the exact same object, like this:

let myMap = new Map();
var obj = {type : 'sum', val : 10};
myMap.set(obj,1);
console.log(myMap.get(obj));