Map set

JavaScript The object of （object）, It is essentially a set of key value pairs , But traditionally, only strings can be used as key value pairs .

To solve this problem ,ES6 Provides map data structure . Its whisper object , It's also a set of key value pairs . But the scope of this key is not limited to strings , Various types of values （ Including objects ） Can be used as a key . in other words object Structure provides （ character string - value ） Corresponding .

ES6 Medium map Type is an ordered list that stores many key value pairs , The key name and corresponding value support all data types . Numbers 5 And string ‘5’ Will be judged as two types , It can appear in the program as two independent keys .

1、 attribute ：size

return map The number of elements

 

2、 The basic method

（1）set()

to map Add data , Returns the added map（ Assigning a value to an existing key will overwrite the previous value ）

（2）get()

Get a key Value , return key Corresponding value , No return undefined

（3）has()

Detect if there is a key, The return Boolean value returns true, No return false

（4）delete()

Delete some key And the corresponding value, Returns a Boolean value , If it is successfully deleted, it will be true

（5）clear()

Clear all values , return undefined

3、 Traverse the way

Be careful ：map The traversal order of is the insertion order , All have MapIterator (Map iterator )

 let map = new Map();
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');

（1）keys()

obtain map All of the key

 console.log(map.keys())// Print all keys

/* Output ：

[Map Iterator] { 'JacksonWang', 'LEO', 'Z-' }

（2）values()

obtain map All values

 console.log(map.values())// Print all the values

/* Output ：

[Map Iterator] { '123', '456', '789' }

(3)entries()

obtain map All members of

console.log(map.entries())// In the form of key value pairs

/* Output ：

 [Map Entries] {

  [ 'JacksonWang', '123' ],

  [ 'LEO', '456' ],

  [ 'Z-', '789' ]

}

(4) for  of For recycling Map iterator  

You cannot traverse objects , It will prompt that the object is not an iterator , Traversal objects can only use for in

 for  of Can traverse array , The value of the variable inside is each item of the array

(4)forEach()

Traverse map All members of

let map = new Map();
map.set('JacksonWang','123');
map.set('LEO','456');
map.set('Z-','789');

for(const [key,value] of map.entries()){
    console.log(`${key}:${value}`);
}

/* Output :

JacksonWang:123

LEO:456

Z-:789

*/

Popular science knowledge : Whether object or array, or Set Data constructors all have keys()/values()/entries() Method

Turn to array exercises :

map Structure to array deconstruction

let map1 = new Map([
    [1,'One'],
    [2,'Two'],
    [3,'Three']
])

console.log([...map1.keys()]);
console.log(...map1.entries())
console.log([...map1.entries()]);

/* Output :

[ 1, 2, 3 ]

[ 1, 'One' ] [ 2, 'Two' ] [ 3, 'Three' ]

[ [ 1, 'One' ], [ 2, 'Two' ], [ 3, 'Three' ] ]

*/