JS built-in method

Js String method

filter() Filtration method

If the return value is true, keep this value

//  Filtration is greater than 0
jsString = jsString.filter(item => {
    return item > 0
});
//  Find the city of Shandong 
jsString = jsString.filter(item => {
    return item.city === ' Shandong '
});

unshift(param) 

Its parameters will be inserted arrayObject The head of

var arr = new Array()
arr[0] = "George"
arr[1] = "John"
arr[2] = "Thomas"

document.write(arr + "<br />")
document.write(arr.unshift("William") + "<br />")
document.write(arr)

sort() 

Sort

//  If you sort in ascending order 
Array.sort(function(a, b) {
  return a - b
})
//  If sorted in descending order 
Array.sort(function(a, b) {
  return b - a
})

//  according to key sorted 
Array.sort(function(obj1, obj2) {
  let val1 = obj1.key
  let val2 = obj2.key
  return val1 - val2
})

Convert characters to numeric arrays

a="1,2,3,4,5" //  ''.split(',') Returns the [""], Then .map[Number] Returns the [0]
if (a == '') {
    b=[]
} else {
    b=a.split(",") // ["1", "2", "3", "4", "5"]
    b.map(Number) // [1, 2, 3, 4, 5]
}

JavaScript Array includes() Method

let site = ['runoob', 'google', 'taobao'];
 
site.includes('runoob'); 
// true 
 
site.includes('baidu'); 
// false

[1, 2, 3].includes(2);     // true
[1, 2, 3].includes(4);     // false
[1, 2, 3].includes(3, 3);  // false
[1, 2, 3].includes(3, -1); // true
[1, 2, NaN].includes(NaN); // true

Of an object in an array key Objects with equal values are combined into one object , loop Map

//  take id Merge the same 
let chartDatas = this.groupBy(results, item => item.id);

groupBy(array, f) {
        // initialization  Map,  Used to store key value pairs 
        let map = new Map();
        array.forEach(function(obj) {
          // According to the function passed in , Generate a for each object in the array  key value , And will key Objects with equal values are grouped 
          let key = f(obj);
          map.set(key, (map.get(key) || []));
          map.get(key).push(obj);
        });
        return map;
      },

for (var [key, value] of chartDatas) {
    let k = key;
    let v = value;
    console.log('key: ' + k + ' value: ' + v);
}