javascript - Why does !function foo(){} return false and !function foo(){}() return true?

Question

As mentioned, thank you!

Answer 1

The only ones judged as false in

js are the following:

• 0

• NaN

• false

• ''

• null

• undefined

Therefore:

!function foo () {} == !(function foo () {}) == false

!function foo(){}() == !(function foo () { return undefined })() == !undefined == true

Answer 2

I think your question can be changed to !(function foo(){}) returns false, !(function foo(){})() returns true?
Because the code function foo(){}() will return Uncaught when executed SyntaxError: Unexpected token error.

The former is to negate the non-empty string. The Boolean value of the non-empty string is true. After negation, it is naturally false;
The latter is to negate the return value of the self-executing function. The return value is undefined and converted into a Boolean value. is false, and its negation is true.

The above are personal opinions, welcome to discuss. Thanks!