{} - 0 VS ({} - 0) 在 JavaScript 中 [英] {} - 0 VS ({} - 0) in JavaScript

问题描述

在 Chrome JavaScript 控制台中，为什么将语句 {} - 0 括在括号中会改变返回值?

In the Chrome JavaScript console, why does wrapping the statement {} - 0 in parentheses change the returned value?

{} - 0    // Returns -0
({} - 0)  // Returns NaN

将单个语句括在括号中会改变包含的值，这似乎非常奇怪.我在这里错过了什么?

It seems incredibly strange that wrapping a single statement in parentheses alters the contained value. What am I missing here?

推荐答案

{} - 0 行有两种可能的解释:

There are two possible interpretations of the line {} - 0:

1. 可以理解为{};-0，其中 {} 被解释为空块语句，- 是一元否定运算符(所以 -0只是负零").求值时 this 的值是最后一条语句的值，即 -0.
2. 可以解释为({} - 0)，其中{}被解释为空对象，-是减法运算符(因此从 {} 中减去 0).

1. It can be interpreted as {}; -0, where {} is interpreted as an empty block statement, and - is the unary negation operator (so -0 is just "negative zero"). The value of this when evaluated is the value of the last statement, which is -0.
2. It can be interpreted as ({} - 0), where {} is interpreted as an empty object, and - is the subtraction operator (so 0 is subtracted from {}).

在您的第一行中，这是模棱两可的，因此它将选择第一种解释.在第二行中，第一个解释是无效的(因为块语句永远不能成为表达式的一部分，你用括号强制它).

In your first line, this is ambiguous, so it will choose the first interpretation. In the second line, the first interpretation is invalid (as a block statement can never be part of an expression, which you're forcing with the parantheses).

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