使用[]运算符时，在Typescript 1.6.2中从内置数组扩展的类不会更新长度 [英] Class extended from built-in Array in Typescript 1.6.2 does not update length while using [] operator

问题描述

当我读到这里：

TypeScript 1.6增加了对扩展任意表达式的类的支持，这些类计算构造函数。这意味着现在可以在类声明中扩展内置类型。

TypeScript 1.6 adds support for classes extending arbitrary expression that computes a constructor function. This means that built-in types can now be extended in class declarations.

...

一些例子：

// Extend built-in types

class MyArray extends Array<number> { }
class MyError extends Error { }

...

但是在扩展Array时，使用[]运算符设置值时不会更新length属性。推送功能工作正常，但我需要[]运算符。

However when extending Array, length property is not updated while using [] operator to set values. Push function works fine, but I need [] operator.

我的例子：

class ExtendedArray<T> extends Array<T> {}

var buildinArray = new Array<string>();
var extendedArray = new ExtendedArray<string>();

buildinArray.push("A");
console.log(buildinArray.length); // 1 - OK
buildinArray[2] = "B";
console.log(buildinArray.length); // 3 - OK

extendedArray.push("A");
console.log(extendedArray.length); // 1 - OK
extendedArray[2] = "B";
console.log(extendedArray.length); // 1 - FAIL
console.dir(extendedArray); // both values, but wrong length

我做错了什么？问题出在哪里？

Am I doing something wrong? Where is the problem?

推荐答案

您的代码转换为此JavaScript代码：

Your code translates to this JavaScript code:

var __extends = (this && this.__extends) || function (d, b) {
    for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p];
    function __() { this.constructor = d; }
    d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
};
var ExtendedArray = (function (_super) {
    __extends(ExtendedArray, _super);
    function ExtendedArray() {
        _super.apply(this, arguments);
    }
    return ExtendedArray;
})(Array);
var buildinArray = new Array();
var extendedArray = new ExtendedArray();
buildinArray.push("A");
console.log(buildinArray.length); // 1 - OK
buildinArray[2] = "B";
console.log(buildinArray.length); // 3 - OK
extendedArray.push("A");
console.log(extendedArray.length); // 1 - OK
extendedArray[2] = "B";
console.log(extendedArray.length); // 1 - FAIL
console.dir(extendedArray); // both values, but wrong length

[ Playground ]

问题是括号表示法不是作为复制数组原型的一部分而复制的：

The problem is that the bracket notation is not copied as part of copying of Array prototype:

在JavaScript中，我们可以通过扩展
本机原型来对本机数据类型进行子类化。这与本机String对象完美配合;
但是，当谈到本机阵列时，事情并不能很好地工作。如果我们扩展Array原型，我们继承本机数组
函数;但是，我们不再能够使用括号表示法
来设置和获取给定数组中的索引值。当然，我们可以使用
push（）和pop（）来克服这个限制;但是，如果我们想保持
括号表示功能的功能，我们必须在
现有数组实例的基础上构建，而不是真正对Array
对象进行子类化。

In JavaScript, we can sub-class native data types by extending the native prototypes. This works perfectly with the native String object; but, when it comes to native Arrays, things don't work quite so nicely. If we extend the Array prototype, we inherit the native array functions; but, we no longer have the ability to use bracket notation to set and get indexed values within the given array. Sure, we can use push() and pop() to overcome this limitation; but, if we want to keep the bracket notation feature functional, we have to build on top of an existing array instance rather than truly sub-classing the Array object.

[来源]

有一个深入的讨论和解决方案。

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