ES6学习——类语法：继承内置的类（如Array，Error等）

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ES6的规范中在介绍内置类时，明确说明了哪些内置类可以subclass，没有说的自然不行。

我们看几个内置类的描述：

The Error constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the specified Error behaviour must include a super call to the Error constructor to create and initialize subclass instances with a [[ErrorData]] internal slot.

The Array constructor is designed to be subclassable. It may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit the exotic Array behaviour must include a super call to the Array constructor to initialize subclass instances that are exotic Array objects.

不再列举了，以前由于Error类不能subclass，所以不能抛出自定义的Error，在ES6中已经可以了。注意这个特性需要浏览器的原生支持，无法用ES6 transpilers实现。

看个数组的例子：

class MyArray extends Array {constructor(len) {super(len);}sum(){return this.reduce(function(prevVal,curVal){return prevVal + curVal},0);}}var arr = new MyArray(3);console.log(typeof MyArray,arr instanceof MyArray,arr instanceof Array);//function true trueconsole.log(arr.length);//3arr[0] = 1;arr[4] = 2;console.log(arr.length,arr);//5 [1,,,,2]console.log(arr.sum());//3

下面在看一个Error的例子：

class RuntimeError extends Error{constructor(desc,funcName) {super(desc);this.funcName = funcName;}toString(){return this.funcName + "->" + this.message}}function test(){throw new RuntimeError("oh~","test");}try{test();}catch(e){console.error(e.toString());//test->oh~}

看了这两个例子是不是觉得JS也开始OO了起来，顿时高大上了。

*以上全部代码在Chrome 47下通过测试

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