ES6:Map

Map object

1、Map object

Map Object to save key value pairs . Any value ( Object or original value ) Can be used as a key or as a value .

2、Maps and Objects The difference between

• One Object The key of can only be a string or Symbols, But a Map The bond of can be any value .
• Map The key values in are ordered （FIFO principle ）, The key added to the object is not .
• Map The number of key value pairs can be obtained from size Property acquisition , and Object The number of key value pairs can only be calculated manually .
• Object They all have their own prototypes , The key name on the prototype chain may conflict with the key name you set on the object .

3、Map Medium key

（1）key Is string

var myMap=new Map();
var kk='a string';
myMap.set(kk,"for a thing");
console.log(myMap.get(kk))//for a thing
console.log(myMap.get("a string"))//for a thing

（2）key It's the object

var myMap=new Map();
var keyObj = {
    }
myMap.set(keyObj,"for a thing");
console.log(myMap.get(keyObj))//for a thing
console.log(myMap.get({
    }))//undefined

（3）key Is the function

var myMap=new Map();
var keyFunc=function () {
    }
myMap.set(keyFunc,"for a thing");
console.log(myMap.get(keyFunc))//for a thing
console.log(myMap.get(function(){
    }))//undefined

（4）key yes NaN

var myMap=new Map();
myMap.set(NaN,"for a thing");
console.log(myMap.get(NaN))//for a thing

var otherNaN=Number("you")
console.log(myMap.get(otherNaN))//for a thing

var myMap=new Map();
myMap.set(NaN,"for a thing");
console.log(myMap.get(NaN))//for a thing

var otherNaN=Number(2,4)
console.log(myMap.get(otherNaN))//undefined

4、Map Iteration

var myMap=new Map();
myMap.set(0,"def")
myMap.set(1,"rfg")myMap.set(3,"ppp")

for(var [index,value] of myMap){
        
	console.log(index+"="+value)
}

var myMap=new Map();
myMap.set(0,"def")
myMap.set(1,"rfg")myMap.set(3,"ppp")

for(var [index,value] of myMap.entries()){
    
    	console.log(index+"="+value)
 }

This entries Method returns a new one Iterator object , It contains... In insertion order Map Of each element in the object [index, value] Array .

var myMap=new Map();
myMap.set(0,"def")
myMap.set(1,"rfg")
myMap.set(3,"ppp")

for (var key of myMap.keys()) {
      
 console.log(key);
}

This keys Method returns a new one Iterator object , It contains... In insertion order Map The key of each element in the object .

var myMap=new Map();
myMap.set(0,"def")
myMap.set(1,"rfg")
myMap.set(3,"ppp")

for (var value of myMap.values()) {
    
  console.log(value);
 }

This values Method returns a new one Iterator object , It contains... In insertion order Map The value of each element in the object .

var myMap=new Map();
myMap.set(0,"def")
myMap.set(1,"rfg")
myMap.set(3,"ppp")
myMap.forEach(function(value,key){
    
    console.log(key+"="+value);
},myMap);

5、Map Operations on objects

（1）Map And Array Transformation

var k=[["one","thr"],["two","yht"]]
var myMap=new Map(k);
var arr=Array.from(myMap)
console.log(arr)

Map A constructor can A two-dimensional The key value is converted to an array Map Object use Array.from Function can Map Object to an array of two-dimensional key value pairs

（2）Map The clone

var myMap1 = new Map([["key1", "value1"], ["key2", "value2"]]);
var myMap2 = new Map(myMap1);
console.log(myMap2===myMap1)//false

//Map  The object constructor generates an instance , Iterate over new objects .
console.log(myMap2)

（3）Map The merger of

var first = new Map([[1, 'one'], [2, 'two'], [3, 'three'],]);
var 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 corresponding value is  uno,dos, three
var merged = new Map([...first, ...second]);