使用 new Map() 映射单个键多个值 - javascript

Question

I need to map single key with multiple values using Map(). But my values keep on getting overwritten. Help.

 var myMap = new Map(); myMap.set("1","A"); myMap.set("1","B");

Answer 1

 var myMap = new Map();

 myMap.set("1",["A"]);
 myMap.set("1",["B", ...myMap.get('1')]);

Answer 2

You could take a Set for multiple values for a key of a map.

 function setValue(map, key, value) { if (!map.has(key)) { map.set(key, new Set(value)); return; } map.get(key).add(value); } var myMap = new Map(); setValue(myMap, "1", "A"); setValue(myMap, "1", "B"); console.log(Array.from(myMap.entries(), ([k, v]) => [k, [...v]]));

Answer 3

A Map data structure is a key/value store. A single key maps to a single value. So you must change the value if you want multiple:

var myMap = new Map();

myMap.set("1",["A","B"]);

Use an array of values, like above, in order to have a Map use a the key "1" to map to multiple value.

The second call to set in the OP, will overwrite the first value.

Answer 4

You probably want the value to be an array. If you want to alter the value one at a time like you example, you need to get the value and then alter it.

 var myMap = new Map(); myMap.set("1", []) //<-- '1' is now an empty array myMap.get("1").push("A"); // <-- now add to the array myMap.get("1").push("B"); console.log(myMap.get("1")) console.log(myMap.get("1")[1]) // <-- just 'B'

Of course, there are lot of other ways of getting the values into the map.

Answer 5

1. Starting with a map object m I iterate over all my key, value pairs.
2. For each value, I also append items already stored in the map object under the same key. m.set([value ... m.get(key)])
3. If these do not exist, and return undefined, I then replace them with with a blank array m.get(key) || [] m.get(key) || []

const keyval = [[2,3],[3,5],[3,8]]

m =  new Map(); keyval.forEach(d=> m.set(d[0],[d[1],...m.get(d[0])||[]]));

returns:

Map(2) {2 => Array(1), 3 => Array(2)}

Answer 6

You have two options. Define an array for property:

let myObj = {key1: ["value1", "value2"]};

or assign them dynamically:

let myObj = {key1: "value2"};
myObj.key2 = ["value1", "value2"];

Answer 7

You can't do this in JavaScript until you have a unique key in that particular object.

Valid:

 {
    a: "1a23",
    b: {
        a: "23gh"
    },
    c: {
        a: 34
    }
 }

Not Valid:-

{
    a: "1a23",
    a: "4354" //Not valid
    b: {
        a: "23gh",
        a: "sdfd" //Not valid
    },
    c: {
        a: 34
    }
 }

Cause if you try to get the property value of "a" how does the compiler figures out which is the one you are requesting. hence java Script overwrites if you have same property or if you try manually assigning it like this

let k = {
a: 1,
a:2
}

It will throw an error. so alternatively you can use an array to do this something like

let k = {
a: [1,2]
}
let k2 = myMap.set("a",[1,2]);