无论React还是RN都已经迈入了ES6的时代，甚至凭借Babel的支持都进入了ES7。ES6内容很多，本文主要讲解类相关的内容。
构造函数
定义侦探类作为例子。
ES5的“类”是如何定义的。
function ES5Detective() {
console.log('##ES5Detective contructor');
}
ES6定义类：
class ES6Detective {
constructor() {
console.log('Detective constructor');
}
}
ES6使用了class关键字，而且有专门的constructor。ES5里的function ES5Detective既是类的定义，也是构造函数。
属性
看看这个侦探是从哪本书出来的。
ES5：
ES5Detective.prototype.fromBookName = 'who';
ES6：
class ES6Detective {
detectiveName: string;
_bookName: string;
constructor() {
console.log('Detective constructor');
this.detectiveName = 'Detective who'; // 属性
}
}
ES6 getter & setter
class ES6Detective {
detectiveName: string;
_bookName: string;
constructor() {
console.log('Detective constructor');
this.detectiveName = 'Detective who';
this._bookName = 'who';
}
get fromBookName() {
return this._bookName;
}
set fromBookName(value) {
this._bookName = value;
}
}
如果只有getter没有setter而赋值的话就会出现下面的错误：
detective.bookAuthor = 'A C';
^
TypeError: Cannot set property bookAuthor of #<ES6Detective> which has only a getter
实例方法
侦探是如何解决案件的。
ES5：
ES5Detective.prototype.solveCase = function(caseName) {
var dn = this.dectiveName;
if(!caseName) {
console.log('SOLVE CASE: ' + dn + ' no case to solve');
} else {
console.log('SOLVE CASE: ' + dn + ' get case ' + caseName + ' is solved');
}
};
或者：
function ES5Detective() {
this.dectiveName = 'Detective who';
console.log('##ES5Detective contructor');
// 实例方法
this.investigate = function(scene) {
console.log('investigate ' + scene);
}
this.assistant = "assistant who";
}
ES6:
class ES6Detective {
detectiveName: string;
_bookName: string;
constructor() {
console.log('Detective constructor');
this.detectiveName = 'Detective who';
this._bookName = 'who';
}
solveCase(caseName) {
if(!caseName) {
console.log('no case to solve');
} else {
console.log('case ' + caseName + ' is solved');
}
}
}
ES6添加方法非常简单直接。ES5中添加实例方法有两种方法，一是在prototype里定义，一是在构造函数重定义。在构造函数中定义的实例方法和属性在每一个实例中都会保留一份，而在原型中定义的实例方法和属性是全部实例只有一份。
另外，在ES5的构造函数重定义的实例方法可以访问类的私有变量。比如：
function ES5Detective() {
console.log('##ES5Detective contructor');
var available: boolean = true; // private field. default income is ZERO.
this.investigate = function(scene) {
if (available) {
console.log('investigate ' + scene);
} else {
console.log(`i'm not available`);
}
}
}
在其他的方法访问的时候就会报错。
if (!available) {
^
静态方法
ES5：
ES5Detective.countCases = function(count) {
if(!count) {
console.log('no case solved');
} else {
console.log(`${count} cases are solved`);
}
};
类名后直接定义方法，这个方法就是静态方法。
ES5Detective.countCases();
ES6:
class ES6Detective {
static countCases() {
console.log(`Counting cases...`);
}
}
// call it
ES6Detective.countCases();
继承
ES6使用extends关键字实现继承。
ES5:
function ES5Detective() {
var available: boolean = true; // private field.
this.dectiveName = 'Detective who';
console.log('##ES5Detective contructor');
this.investigate = function(scene) {
// 略
}
this.assistant = "assistant who";
}
ES5Detective.prototype.solveCase = function(caseName) {
// 略
}
// inheritance
function ES5DetectiveConan() {
// first line in constructor method is a must!!!
ES5Detective.call(this);
this.dectiveName = 'Conan';
}
// inheritance
ES5DetectiveConan.prototype = Object.create(ES5Detective.prototype);
ES5DetectiveConan.prototype.constructor = ES5DetectiveConan;
ES5继承的时候需要注意两个地方：
- 需要在子类的构造函数里调用
SuperClass.call(this[, arg1, arg2, ...])子类的prototype赋值为：
SubClass.prototype = Object.create(SuperClass.prototype)，然后把构造函数重新指向自己的：SubClass.prototpye.constructor = SubClass。ES6：
class ES6Detective {
constructor() {
console.log('Detective constructor');
this.detectiveName = 'Detective who';
this._bookName = 'who';
}
solveCase(caseName) {
if(!caseName) {
console.log('no case to solve');
} else {
console.log('case ' + caseName + ' is solved');
}
}
get fromBookName() {
return this._bookName;
}
set fromBookName(value) {
this._bookName = value;
}
get bookAuthor() {
return 'Author Who';
}
static countCases() {
console.log(`Counting cases...`);
}
}
class ES6DetectiveConan extends ES6Detective {
constructor() {
super();
console.log('ES6DetectiveConan constructor');
}
}
ES6的新语法更加易懂。
注意：一定要在子类的构造方法里调用super()方法。否则报错。
调用super类内容
class ES6DetectiveConan extends ES6Detective {
constructor() {
super();
console.log('ES6DetectiveConan constructor');
}
solveCase(caseName) {
super.solveCase(caseName);
if(!caseName) {
console.log('CONAN no case to solve');
} else {
console.log('CONAN case ' + caseName + ' is solved');
}
}
}
静态方法可以被继承
ES6的静态方法可以被继承。ES5的不可以。
class ES6Detective {
static countCases(place) {
let p = !place ? '[maybe]' : place;
console.log(`Counting cases...solve in ${p}`);
}
}
class ES6DetectiveConan extends ES6Detective {
constructor() {
super();
console.log('ES6DetectiveConan constructor');
}
}
// static method
ES6Detective.countCases();
ES6DetectiveConan.countCases('Japan');
// result
Counting cases...solve in [maybe]
Counting cases...solve in Japan
在子类ES6DetectiveConan并没有定义任何方法，包括静态方法。但是，在父类和子类里都可以调用该方法。
甚至，可以在子类里调用父类的静态方法：
class ES6DetectiveConan extends ES6Detective {
static countCases(place) {
let p = !place ? '[maybe]' : place;
super.countCases(p);
console.log(`#Sub class:- Counting cases...solve in ${p}`);
}
}
// result
Counting cases...solve in [maybe]
Counting cases...solve in Japan
#Sub class:- Counting cases...solve in Japan
代码
https://github.com/future-challenger/ES-Samples/tree/master/class
