ES6 extensions to arrays

spread operator...

Spread operator ...converts an array to a comma-separated sequence of arguments

...Can be followed by an expression that returns an array
...If the latter is an empty array, it will have no effect

let m = 2,
  arr = [...(m > 1 ? [1, 2] : [3, 4])];

function fn(a, b, c) {
  return a + b + c;
}
console.log(fn(5, ...arr)); // 8

application

Alternative to appplay

// 求数组最大值

// ES5
Math.max.apply(null, [5, 3, 7]);
// ES6
Math.max(...[5, 3, 7])

copy array

make a full copy, not a reference to the array

let arr = [3, 2, 5];

/* ES5 */
let a = arr.concat();

/* ES6 */
// 写法一
let b = [...arr];
// 写法二
let [...c] = arr;

console.log(a);
console.log(b);
console.log(c);

Merge arrays

let a = [1, 2],
  b = [3],
  c = [4, 5];

// ES5
let m = a.concat(b, c);

// ES6
let n = [...a, ...b, ...c];
console.log(m);
console.log(n);

Combined with destructuring assignment

If the spread operator is used for assignment, it can only be placed at the end, otherwise an error will be reported

let [a,...b] = [1,2,3];
console.log(a); // 1
console.log(b); // [2,3]

// let [...b,a] = [1,2,3];  // 报错
let [m,...n] = [];
console.log(m); // undefined
console.log(n); // []

Convert an object with Iterator interface to an array

let a = [...'hello'];
console.log(a); // ["h", "e", "l", "l", "o"]

// querySelectorAll 方法返回的是 nodeList 对象，这个对象是类数组而不是真的数组，扩展运算符可将它转换为数组
let dvs = document.querySelectorAll('div'),
arr = [...dvs];

find，findIndex

find(fn,this), this is optional, it is the this of fn, execute fn on each item of the array until the first item whose return value is true is found, and return the item, if no matching condition is found item returns undefined

findIndex is similar to find, returns the position of the first item that matches the condition, and returns -1 if none of the items match

fill

Array.from(a,fn)

Any object with a length property can be converted to an array by Array.from(), such as nodeList, arguments
a is a true array, returns an identical new array

Array.of()

Array.of() can receive multiple parameters, which are used as array members to generate an array

copyWithin()

Receives three parameters, overlays the members at the specified position of the array with the members at another position, and returns the array
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