First, the built-in class in JS

Common built-in class in JS

1. Every data type has its own built-in class
   Number、String、Boolean、Symbol、BigInt
   Object、Function、Array、Date、RegExp、Error(ReferenceError/TypeError/RangeError/SyntaxError)…
2. Each element object also has its own built-in class
3. The rest of the built-in class
   XMLHttpRequest、Promsie、Proxy…

Custom class "class-> constructor"
ES5: Create a function, perform a function based on "New", this function is called the constructor "class", the result is an instance of the class
ES6: Create a custom class based on "Class"

The difference between ordinary function execution and new execution functions

JS code runtime:

1. Create an EC (G) in the store - Global execution context
2. Create VO (G) / GO in EC (G), store global variables, and improve variables
3. Open an address storage function plot in the heap memory, the storage function scope in the heap, the code string and function in the function (Name, Length)
4. Code execution

Ordinary function execution:

• Create an EC (Fn) - function execution context in the stack memory
• Create AO (FN) in EC (FN), store function private variables
• Function execution: Initialization scope chain, initial THIS, initial arguments, meticulum assignment, variable lifting, code execution

NEW execution function: (execute the function as constructor)

• Create an EC (Fn) - function execution context in the stack memory
• Create AO (FN) in EC (FN), store function private variables
• An instance object that creates an empty object (FN) by default
• Function execution: Initialization scope chain
• Initial THIS, let this default to the instance objects previously created
• Initial arguments, meticulum assignment, variable lifting
• Code Execution: In the post-function code execution, this.xxx = xxx appears is a private property and method set to instance objects.
• If the return value is not set in the function (or the return value is the value of the original type), the default returns "instance object", only the value of the object type is the value of the object type, is mainly

example:

function Fn(x, y) {
    let sum = 10;
    this.total = x + y;
    this.say = function () {
        console.log(`I calculate and is:${this.total}`);
    };
}
// let res = fn (10, 20); // Normal function execution => FN function RES function return value
// console.log(res); //undefined
// console.log (window.total, window.say, window.sum); // 30 functions undefined

let f1 = new Fn(10, 20); // Constructor execution => FN constructor (class) F1 class instance
console.log(f1); // Example Object Total & Say