Array.apply(), new Array(), arr," the difference between the three

Why would you write this article? L ooking at the Vue document 渲染函数 when you find a problem curious, Array.apply(null, { length: 20 }) Why define arrays like this? Then look up the data to make a small knot record, sparrows are small, dirty all.

Array.apply()

Apply() is explained in MDN as follows:

func.apply(thisArg, [argsArray])

ThisArg is a must. T he value of this used when the func function runs. Note that this may not be the actual value that the method sees: if the function is in non-strict mode, it is automatically replaced with a global object when specified as null or undefined, and the original value is wrapped.

argsArray optional. A n array or class array object in which array elements are passed to the func function as separate arguments. I f the value of the argument is null or undefined, no arguments need to be passed in. Class array objects can be used from ECMAScript 5

function printArguments() {
    console.log(arguments)
    Array.prototype.forEach.call(arguments, function (item) {
        console.log(item);
    });
}
printArguments(undefined,undefined)

From this, it can be seen that {length:20} class array such as argument only the property of length which is equivalent to creating an array of 20 lengths, each element undefined 类数组

(20) [undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined, undefined]

new Array()

new Array(20) and Array(20) simply create an array of 20 lengths and the elements are empty

(20) [empty × 20]

arr = []

let arr=[];
arr.length= 20
(20) [empty × 20]

From this you can see new Array(20) and let arr=[];arr.length= 20

Array.from()

The Array.from() method creates a new, shallow-copy array instance from an array-like or iterative object.

Array.from({length:20})
(20) [undefined, undefined, undefined, undefined, undefined, undefined, undefi

Source: Public Number - Clown's Cabin Author: Clown

The above is a description of the difference W3Cschool编程狮 and Array.apply(), new Array(), arr., and I hope to help you.