Is null considered zero and undefined not a number on arithmetic expressions?

问题

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

According to some testing it seems so:

> null + null
0

> 4 + null
4

> undefined + undefined
NaN

> 4 + undefined
NaN

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

回答1:

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

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

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

It is used in the following "arithmetic" expressions:

• 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

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

回答2:

It seems safe to assume so since, in an arithmetic expression (e.g. addition), the method ToNumber would be called on it, evaluating NaN and +0 from undefined and null respectively:

                     To Number Conversions
╔═══════════════╦════════════════════════════════════════════╗
║ Argument Type ║                   Result                   ║
╠═══════════════╬════════════════════════════════════════════╣
║ Undefined     ║ NaN                                        ║
║               ║                                            ║
║ Null          ║ +0                                         ║
║               ║                                            ║
║ Boolean       ║ The result is 1 if the argument is true.   ║
║               ║ The result is +0 if the argument is false. ║
║               ║                                            ║
║ Number        ║ The result equals the input argument (no   ║
║               ║ conversion).                               ║
║               ║                                            ║
║ String        ║ See grammar and note below.                ║
║               ║                                            ║
║ Object        ║ Apply the following steps:                 ║
║               ║   1. Let primValue be ToPrimitive(input    ║
║               ║      argument, hint Number).               ║
║               ║   2. Return ToNumber(primValue).           ║
╚═══════════════╩════════════════════════════════════════════╝

ECMAScript Language Specification - ECMA-262 Edition 5.1

回答3:

Without being type bound,

null == false == 0

null !== false !== 0

http://www.mapbender.org/JavaScript_pitfalls:_null,_false,_undefined,_NaN#0_6

With that said, null == 0, null + 4 = 4

I hope this helps.

来源：https://stackoverflow.com/questions/21001320/is-null-considered-zero-and-undefined-not-a-number-on-arithmetic-expressions