八个falsy变量，用于if判断或者逻辑符
There are 8 falsy values:
false |
The keyword false |
0 |
The number zero |
-0 |
The number negative zero |
0n |
BigInt, when used as a boolean, follows the same rule as a Number. 0n is falsy. |
"" |
Empty string value |
| null | null - the absence of any value |
| undefined | undefined - the primitive value |
| NaN | NaN - not a number |