Detailed explanation of Map and introduction to commonly used APIs

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The Map type in ECMAScript 6 is an ordered list that stores many key-value pairs. Key-value pairs support all data types. Keys 0 and '0' will be treated as two different keys, and no cast conversion will occur.

How to use Map?

let map = new Map();

Common methods:

set(key, value): Add a new key-value pair element

get(key): Get the value corresponding to the key. If the value does not exist, return undefined

let map = new Map();
map.set( '0', 'ghostwu' );
map.set( 0, 'ghostwu' );

console.log( map.get( '0' ) ); //ghostwu
console.log( map.get( 'name' ) ); //undefined;

let map = new Map();
var key1 = {}, key2 = {};

map.set( key1, 'ghostwu' );
map.set( key2, 22 );

console.log( map.get( key1 ) ); //ghostwu
console.log( map.get( key2 ) ); //22

Okay Use the object as the key of the Map. Although there are two empty objects, strong type conversion will not occur.

has( key): Determine whether the key name exists

delete( key): Delete key names and corresponding values

clear(): Remove all key-value pairs in the map collection

size: The number of elements in the map collection

let map = new Map();
map.set( 'name', 'ghostwu' );
map.set( 'age', 22 );

console.log( map.has( 'name' ) );//true
console.log( map.size ); //2

map.delete( 'name' );
console.log( map.has( 'name' ) );//false
console.log( map.size ); //1
console.log( map.has( 'age' ) ); //true

map.clear();
console.log( map.size ); //0
console.log( map.has( 'age' ) ); //false

Map supports array initialization, using a two-dimensional array, each array uses key-value pairs

let map = new Map( [ [ 'name', 'ghostwu' ], [ 'age', 22 ] ] );
console.log( map.has( 'name') ); //true
console.log( map.has( 'age') ); //true
console.log( map.size ); //2
map.set( 'sex', 'man' );
console.log( map.size );
console.log( map.get( 'name' ) ); //ghostwu
map.clear();
console.log( map.size ); //0

Map also supports the forEach method, support 2 parameters, the first one: function, the function supports 3 parameters (value, key, current map), the second one: this

let map = new Map( [ [ 'name', 'ghostwu' ], [ 'age', 22 ] ] );
map.set( 'sex', 'man' );
map.forEach( function( val, key, cur ){
 console.log( val, key, cur, this );
}, 100 );

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