p82.
书中：条件判断a==null仅在。。。。。。
原文：
The a == null check will pass only if doSomething() returns either null or undefined, and will fail with any other value, even other falsy values like 0, false, and "".
https://github.com/getify/You-Dont-Know-JS/blob/master/types%20%26%20grammar/ch4.md