map():

Different from forEach and other traversal methods, the return statement has no effect in forEach. Map can change the value of the current loop and return a new array after the changed value (map needs return), which is generally used to process the value of an array to be modified.

let arr1 = [1,2,3];
let arr2 = arr1.map((value,key,arr) => {
    console.log(value)    // 1，2，3
    console.log(key)      // 0，1，2
    console.log(arr)      //[1,2,3] [1,2,3] [1,2,3]
    return value * value;
})
console.log(arr1); // [ 1, 2, 3 ]
console.log(arr2); // [ 1, 4, 9 ]

filter():

The filter function can be seen as a filter function that returns an array of qualified elements

The filter needs to judge whether it is true or false when it is looping. Only true can return this element;

 

let arr1 = [1,2,3];
let arr2 = arr1.filter((value,key,arr) => {
    console.log(value)    // 1，2，3
    console.log(key)      // 0，1，2
    console.log(arr)      // [1,2,3]
    return value >= 3 ? false : true;     
})
console.log(arr1); // [ 1, 2, 3 ]
console.log(arr2); // [ 1, 2 ]

filter() filters non conformances

let arr = [1,2,3]
let newArr = arr.filter(item => item>=3)  
console.log(newArr)

filter() removes empty string, undefined, null

let arr = ['','1','2',undefined,'3.jpg',undefined]
let newArr = arr.filter(item => item)
console.log(newArr)

filter() array de duplication

let arr = [1, 2, 2, 3, 4, 5, 5, 6];
let newArr = arr.filter((x, index,self)=>self.indexOf(x)===index) 
console.log(newArr)

set() method is recommended for de duplication

let arr = [1, 2, 2, 3, 4, 5, 5, 6];
let newArr =Array.from(new Set(arr)) 
console.log(newArr)

filter() filters array objects

Single condition filter:

let arr = [
	    {a:'Apple',b:'bread',c:'eat'},
	    {a:'Banana',b:'bread',c:'Not to eat'},
	    {a:'Banana',b:'Apple',c:'eat'},
	    {a:'Apple',b:'Banana',c:'Not to eat'},
	  ]
console.log(arr.filter(item => item.a=== 'Apple' ))//[{a: 'Apple', b: 'bread', c: 'eat'}, {a: 'Apple', b: 'Banana', c: 'don't eat'}]

Multiple criteria screening: at present, only this stupid method is thought of

let a = 'Apple'; //Filter parameter a
let b = 'bread'; //Filter parameter b
let c = ''     //Filter parameter c
let arr = [
	    {a:'Apple',b:'bread',c:'eat'},
	    {a:'Banana',b:'bread',c:'Not to eat'},
	    {a:'Banana',b:'Apple',c:'eat'},
	    {a:'Apple',b:'Banana',c:'Not to eat'},
	  ];
if (a != '') {
    arr = arr.filter(item => item.a === a)
}
if (b != '') {
    arr = arr.filter(item => item.b === b)
}
if (c != '') {
    arr = arr.filter(item => item.c === c)
}
console.log(arr) // Screening results: [{a: 'Apple', b: 'bread', c: 'eat'}]

Change from: https://blog.csdn.net/zuorishu/article/details/80958705

Related links: https://juejin.im/post/5b5a9451f265da0f6a036346

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