这是什么== - javascript运算符？ [英] What is this ==- javascript operator?

问题描述

当我发现 == - 或 == + 时，我一直在尝试不同的条件。 / p>

在JS控制台中，你可以写：

  var a = ; 
  

然后以下是真的

  a ==; 
  

但这是假的

  a ==; 
  

但是，如果你说：

  a ==  - ; 
  

或

  a == +; 
  

那么漂亮的 == - 运算符？

解决方案

它们不是不同的运算符。

写作：

  a ==  - ; 
  

被解析为：

 （a）==（ - ）; 
  

== + 也是如此。

由于Javascript的怪异类型转换规则，表达式的计算结果为 true 。发生以下情况：

1. 一元 - （或 + ）运算符将其操作数转换为数字。如果它是一个空白字符串，则此转换的结果为 0 。


2. a ==（ - 然后）等同于== 0 。如果与 == 比较的类型不同，则将其中一个（可能两者）转换为获得通用类型。在这种情况下，左侧的也会转换为 0 。


3. 您最终将 0 与 0 进行比较，产生 true 。

（以上是Javascript如何得出这个结果的一个粗略示例，实际规则隐藏在 ECMAScript规范。您可以使用 === 运算符来阻止转换并获取 false 如果比较对象的类型不同。）

I was stumbling around trying different conditions, when I discovered ==-, or ==+.

In a JS console, you can write:

var a = " ";

then the following is true

a == " ";

but this is false

a == "    ";

However, it will be true if you say:

a ==- "   ";

or

a ==+ "   ";

So what is this nifty ==- operator?

解决方案

They're not distinct operators.

Writing:

a ==- " ";

gets parsed as:

(a) == (-" ");

The same goes for ==+.

The expression evaluates to true because of Javascript's weird type conversion rules. Something like the following occurs:

1. the unary - (or +) operators converts its operand to a number. If it's a blank string, the result of this conversion is 0.
2. a == (-" ") is then equivalent to " " == 0. If types compared with == are different, one (possibly both), get converted to get a common type. In this case, the " " on the left-hand side gets converted to 0 too.
3. You end up comparing 0 to 0, which yields true.

(The above is a rough example of how Javascript might come to this result, the actual rules are buried in the ECMAScript specification. You can use the === operator instead to prevent the conversions and get false if the types of the compared objects are different.)

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