TypeScript 中的枚举:JavaScript 代码在做什么? [英] Enums in TypeScript: what is the JavaScript code doing?

问题描述

以下打字稿:

enum PrimaryColors { Red, Green, Blue };

生成以下 JavaScript:

var PrimaryColors;(功能(原色){PrimaryColors[PrimaryColors["Red"] = 0] = "Red";PrimaryColors[PrimaryColors["Green"] = 1] = "Green";PrimaryColors[PrimaryColors["Blue"] = 2] = "Blue";})(PrimaryColors || (PrimaryColors = {}));;

我很尴尬地承认我不明白 JavaScript 在做什么.
括号中的函数是使用另一个分配作为索引/键来分配字符串值.我以前从未见过这样的事情.
函数后面的 (PrimaryColors || (PrimaryColors = {}) 的目的是什么?
如果答案是正确地学习 JavaScript，我会欣然接受它，只要它附带一个建议的来源，清楚地解释我在这里看到的内容.

解决方案

我相信:

PrimaryColors[PrimaryColors["Red"] = 0] = "Red";

相当于:

PrimaryColors[0] = "红色";原色["红色"] = 0;

请参阅此参考.><块引用>

表达式 x = 7 是第一种类型的示例.这个表情使用 = 运算符将值 7 赋给变量 x.这表达式本身的计算结果为 7.

例如:

console.log((x = 7));

输出:

7

同样:

var x = {};console.log((x["hi"] = 7));

也输出 7.

---

至于第二件事，PrimaryColors 最初是未定义的.

var x;控制台日志(x)；//不明确的

在布尔上下文中，undefined 的计算结果为 false:

console.log(!undefined);//真的控制台日志(！！未定义)；//错误的

健全性检查:

console.log((!undefined) === true);//真的console.log((!!undefined) === false);//真的console.log(undefined === false);//错误的

这是短路的常见用法.因为 PrimaryColors 最初是未定义的 (false)，它会将 {} 传递给函数.

PrimaryColors ||(原色 = {})

The following TypeScript:

enum PrimaryColors { Red, Green, Blue };

Produces the following JavaScript:

var PrimaryColors;
(function (PrimaryColors) {
    PrimaryColors[PrimaryColors["Red"] = 0] = "Red";
    PrimaryColors[PrimaryColors["Green"] = 1] = "Green";
    PrimaryColors[PrimaryColors["Blue"] = 2] = "Blue";
})(PrimaryColors || (PrimaryColors = {}));
;

I am embarrassed to admit that I don't understand what the JavaScript is doing.
The function in parentheses is assigning string values using another assignment as the index/key. I have not seen anything like this before.
And what is the purpose of the (PrimaryColors || (PrimaryColors = {}) following the function?
If the answer is to learn JavaScript properly, I will readily accept it, provided it comes with a suggested source that clearly explains what I am seeing here.

解决方案

I believe:

PrimaryColors[PrimaryColors["Red"] = 0] = "Red";

is equivalent to:

PrimaryColors[0] = "Red";
PrimaryColors["Red"] = 0;

See this reference.

The expression x = 7 is an example of the first type. This expression uses the = operator to assign the value seven to the variable x. The expression itself evaluates to seven.

For example:

console.log((x = 7));

outputs:

7

Similarly:

var x = {};
console.log((x["hi"] = 7));

Also outputs 7.

---

As for the second thing, PrimaryColors is initially undefined.

var x;
console.log(x); // undefined

In a boolean context, undefined evaluates to false:

console.log(!undefined); // true
console.log(!!undefined); // false

Sanity check:

console.log((!undefined) === true); // true
console.log((!!undefined) === false); // true
console.log(undefined === false); // false

This is a common usage of short circuiting. Because PrimaryColors is initially undefined (false), it will pass {} to the function.

PrimaryColors || (PrimaryColors = {})

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