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

Question

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

According to some testing it seems so:

Answer 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)!