use! To determine whether data is present

In operation often encountered with ! Determines whether the data exists in the case of, for example: if (! A) {balabala } simply summarize, imperfect later added:

First, based on the type of

　　let a = '';

　　let a = null;

　　let a = undefined;

　　let a = false;

　　let a = 0;

Second, the type can not be determined

　　let a = [];

　　let a = {};

　　let a = 1;

Third, the determination method

　　console.log(!a);