Js--class Foundation

JS class? Actually, it's a prototype!

1 class point{2    Constructor (x, y) {3this         . x = x; 4         this. y = y; 5     }6    toString () {7         return this this. Y + ') '; 8     }9 }

Basic points

1. Class is the keyword, and class point defines a category. Other variable names cannot be the same as the class name

2. Type of point (typeof): function, must be called by new

• Class pototype=> contains all defined methods (including constructor) in class

  Point.protoype; // {constructor:?, toString:?}

3. Like a function, a class can also be defined in the form of an expression

• var MyPoint = class{}; var New MyPoint ();

  No variable promotion: You must define before you can use class

• The Name property (like a function name), similarly, class names can only be used inside the class

  Mypoint.name; // MyPoint var MyPoint2 = class me{}; Mypoint2.name; // MyPoint2 new Me (); // error, me undefined, can only be used inside the class

4. Constructor is a constructor

• An object is instantiated with new point, and the Contructor in class is automatically called
• Return value: Default Return instance object this, can specify other return value (same as ES5)
• Can be omitted? If you do not explicitly write a constructor, an empty (same as C + +) is added automatically
• Point.prototype.contructor is class, not contructor function

• var New Point (); Console.log (point.constructor===point); // true

  The this in constructor points to the instantiated object, as in the ES5

  Console.log (point); // Point {x:undefined, y:undefined}

5. Definition of Object properties: Use this in constructor to add; Definition of object methods: defined directly in class (without function and comma)

6. Methods defined in class are not enumerable

1 object.keys (point.prototype); // NULL result, description not enumerable 2 // Enumerable in Es5 3 function Test () {} 4 function (){}; 5 Object.keys (Test.prototype); // [FUN1] Enumerable

View Code

7. Instance Objects

• Share a prototype object (same as ES5)

• var New Point (); var New  = = = p2.__proto__; // true

  The method of the prototype object can be added directly to the class, if you want to add a property =>getprototypeof/point.prototype

  point.__proto__ = = = Point.prototype; // true var pproto == 1;p oint.newattr; // 1

8. Static methods and properties (Static)

• They belong to class, not attribute instances (same as C + +)
• Static properties can only be added by classes outside of class

• class staticclass{static    constructor () {// Define a static method        Console.log (this) ; // class{...}     }    Constructor () {        console.log (this);   point to instance object      = 1;   Define a static property

  This in the static method points to the class!

9. New properties of New.target

• The function or class is called with new, then returns itself using new.target inside them, otherwise returning undefined
• can be used to distinguish whether a function is called by new

• function Point1 () {    console.log (new. target);} Point1 (); // undefined var New Point1 (); // function Point1 () {...}

  Use in class

  var targetclass = class me{    constructor () {        console.log (newTarget===targetclass ); // true        Console.log (newtarget===me); // true     }}varnew targetclass ();

Js--class Foundation