null是否为零，未定义为算术表达式上的数字？ [英] Is null considered zero and undefined not a number on arithmetic expressions?

问题描述

null 评估为 0 和 undefined to算术表达式 NaN ？

Is null evaluated to 0 and undefined to NaN on arithmetic expressions?

根据一些测试看来似乎如此：

According to some testing it seems so:

> null + null
0

> 4 + null
4

> undefined + undefined
NaN

> 4 + undefined
NaN

假设这样安全或正确吗？ （文档中的引用将是A +）。

Is it safe or correct to assume this? (a quote from a documentation would be A+).

推荐答案

将null计算为0并且未定义算术表达式上的NaN？假设这样安全或正确吗？

Is null evaluated to 0 and undefined to NaN on arithmetic expressions? Is it safe or correct to assume this?

是的，是的。 算术表达式将使用 ToNumber 操作：

Yes, it is. An "arithmetic expression" would use the ToNumber operation:

 Argument Type | Result
 --------------+--------
 Undefined     | NaN
 Null          | +0
 …             |

它用于以下算术表达式：

It is used in the following "arithmetic" expressions:

• 前缀 / postfix 递增和递减


• 一元 + 和 - 运营商


• + operator 如果两个参数都不是字符串


• 减法，乘法，除法和模数操作


• relational 运算符如果不是两个参数都是字符串

• prefix/postfix increment and decrement
• the unary + and - operators
• the + operator if none of the two arguments is a string
• subtraction, multiplication, division and modulo operation
• relational operators if not both arguments are strings

相等运算符，所以 null == 0 是假的（并且 null！== 0 无论如何）！

It is not used by the equality operators, so null == 0 is false (and null !== 0 anyway)!

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