How to determine whether a variable has a value in javascript_javascript skills

The example in this article describes how JavaScript determines whether a variable has a value. Share it with everyone for your reference. The specific analysis is as follows:

For example:

var a = null;
var b = undefined;

The above two situations are counted as having no value

if(a !== null && a !== undefined) {
  //
}
// 为什么不能单用if(a !== null) { // }
var a;
a !== null; // true
// 为什么不能单用if(a !== undefined) { // }
var a = null;
a !=== undefined; // true

I hope this article will be helpful to everyone’s JavaScript programming design.