JavaScript null和plus（+）operatior [英] JavaScript null and plus (+) operatior

问题描述

我想了解JavaScript的核心。我知道它没有太大的实施价值。如果你不想回答，那就离开吧。但是，如果您在应用加法（+）时能够帮助理解以下类型强制，我将不胜感激。

I am trying to understand the core of JavaScript. I know it doesnt have much implementation value. If you dont want to answer, just leave it. However, I will appreciate if you could help to understand the following type coercion while applying addition(+).

1. 
null + null // 0
2.
null + undefined; // NaN
3.
null + NaN; // NaN
4. 
1 + null; //1
5. 
true + null; //1
6. 
true + [null]; //"true"

我知道null是一个空的或丢失的对象。如果你能解释类型强制或一元（+）操作的步骤，我将不胜感激。感谢您阅读此问题。

I know null is an empty or missing object. I will appreciate, if you can explain steps in type coercion or unary(+) operation here. Thanks for reading the question.

推荐答案

这包含在 11.6.1加法运算符（+） - 随意阅读并遵守规则。

This is covered in 11.6.1 The Addition operator ( + ) - feel free to read it and follow the rules.

前五个案例可以是通过查看 ToNumber 来解释：

The first five cases can be explained by looking at ToNumber:

Value       ToNumber(Value)
---------   ---------------
null        0
undefined   NaN
NaN         NaN
1           1
true        1

和 0 + 0 == 0 （及 1 + 0 == 1 ），而 x + NaN 或 NaN + x 评估为NaN 。由于上述每个值也都是原始值，因此 ToPrimitive（x） 计算到x（其中x不是字符串），并且未调用字符串连接子句。

And 0 + 0 == 0 (and 1 + 0 == 1), while x + NaN or NaN + x evaluates to NaN. Since every value above is also a primitive, ToPrimitive(x) evaluates to x (where x is not a string) and the "string concatenation clause" was not invoked.

最后一种情况不同它来自 ToPrimitive （最终调用 Array.prototype.toString ）会产生 string 值。因此它最终应用 ToString ，而不是 ToNumber ，并且如下：

The final case is different in that it results from the ToPrimitive (which ends up calling Array.prototype.toString) on the array which results in a string value. Thus it ends up applying ToString, not ToNumber, and follows as such:

   true + [null]
=> true + ""          // after ToPrimitive([null]) => ""
=> "true" + ""        // after ToString(true) => "true"
=> "true"             // via String Concatenation

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