Some methods for callback functions and arrays

about callback Functions

Pass the callback function callback as a parameter to the asynchronous execution function, and then trigger callback execution when the asynchronous execution function finishes executing.

Defines an asynchronous execution function, a callback function as a parameter function
A (callback) {
    callback ();  
    Console.log (' I am an async execution function ');      
}

Define the callback function
B () {
    setTimeout (' Console.log (' I am a callback function ') ', ' n ');     Mimic time-consuming operation  
}

A (B);

I am the asynchronous execution function
//I am the callback function

The code above illustrates that an asynchronous execution function can execute its own code (asynchronous operation) without waiting for the callback function to finish .

Advantages: Simple, easy to understand and deploy

Cons: not conducive to code reading and maintenance, the various parts are highly coupled, each task can only specify a callback function. Array Method

Array.prototype.map ()
The map () method returns a new array in which each element is the result of a callback function.

var numbers = [1, 4, 9];
var roots = Numbers.map (math.sqrt);     The value of the call
//roots in order is [1, 2, 3], the value of numbers is still [1, 4, 9]

Array.prototype.join ()
The join () method joins all elements of an array (or a class array object) into a string .

Array.join (separator) Separator optional. Specifies the delimiter to use. If this argument is omitted, a comma is used as the delimiter.

var array = [1,2,3,4,5];
var str = array.join ("");
str = "12345", array = [1,2,3,4,5]