[JS -- map string]

Map

Map Object to save key value pairs , And remember the original insertion order of the keys . Any value ( Object or original value ) Can be used as a key or as a value .
Constructors

Map()

establish Map object

attribute

Map.length
attribute length The value of is 0 .
Want to calculate one Map Number of entries in , Use Map.prototype.size.

Example

1. Use Map object

let myMap = new Map();

let keyObj = {
    };
let keyFunc = function() {
    };
let keyString = 'a string';

//  Add key 
myMap.set(keyString, " Sum key 'a string' The value of the Association ");
myMap.set(keyObj, " Sum key keyObj The value of the Association ");
myMap.set(keyFunc, " Sum key keyFunc The value of the Association ");

myMap.size; // 3

//  Read the values 
myMap.get(keyString);    // " Sum key 'a string' The value of the Association "
myMap.get(keyObj);       // " Sum key keyObj The value of the Association "
myMap.get(keyFunc);      // " Sum key keyFunc The value of the Association "

myMap.get('a string');   // " Sum key 'a string' The value of the Association "
                         //  because keyString === 'a string'
myMap.get({
    });           // undefined,  because keyObj !== {}
myMap.get(function() {
    }); // undefined,  because keyFunc !== function () {}

2. take NaN As Map Key

NaN It can also be used as Map Object's key . although NaN Not equal to any value, not even yourself (NaN !== NaN return true), But the following example shows ,NaN As Map There is no difference for the key :

let myMap = new Map();
myMap.set(NaN, "not a number");

myMap.get(NaN); // "not a number"

let otherNaN = Number("foo");
myMap.get(otherNaN); // "not a number"

Use forEach() Method iteration Map

Map It can also be done through forEach() Method iteration :

let myMap = new Map();
myMap.set(0, "zero");
myMap.set(1, "one");
myMap.forEach(function(value, key) {
    
  console.log(key + " = " + value);
})
//  Two will be displayed logs.  One is  "0 = zero"  The other is  "1 = one"

Map Relationships to arrays

let kvArray = [["key1", "value1"], ["key2", "value2"]];

//  Use regular Map The constructor can convert a two-dimensional array of key value pairs into a Map object 
let myMap = new Map(kvArray);

myMap.get("key1"); //  The return value is  "value1"

//  Use Array.from Function can Map Object to an array of two-dimensional key value pairs 
console.log(Array.from(myMap)); //  Output and kvArray Same array 

//  A simpler way to do the same thing as above , Use the expansion operator 
console.log([...myMap]);

//  Or use... On iterators of keys or values Array.from, Then get an array containing only keys or values 
console.log(Array.from(myMap.keys())); //  Output  ["key1", "key2"]

Copy or merge Maps

Map Can be copied like an array ：

let original = new Map([
  [1, 'one']
]);

let clone = new Map(original);

console.log(clone.get(1)); // one
console.log(original === clone); // false.  Shallow comparison   Not a reference to the same object 

Map Objects can be merged , But it will keep the uniqueness of the key .

let first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
]);

let second = new Map([
  [1, 'uno'],
  [2, 'dos']
]);

//  Merge two Map Object time , If there are duplicate key values , Then the latter will cover the former .
//  The expansion operator is essentially to expand Map Object to array .
let merged = new Map([...first, ...second]);

console.log(merged.get(1)); // uno
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three

Map Objects can also be merged with arrays ：

let first = new Map([
  [1, 'one'],
  [2, 'two'],
  [3, 'three'],
]);

let second = new Map([
  [1, 'uno'],
  [2, 'dos']
]);

// Map Object is merged with an array , If there are duplicate key values , Then the latter will cover the former .
let merged = new Map([...first, ...second, [1, 'eins']]);

console.log(merged.get(1)); // eins
console.log(merged.get(2)); // dos
console.log(merged.get(3)); // three