javascript 空与未定义

Posted 2021-05-03

// unndefined - Represents an absence of value. Assigned by the system. Should let unefined just happen.
// null - Represents an absence of valuee. Assigned by the prograam. If you are assigning an absence of value, use null

var a;
// example 1
if(!a){	a = 100;}

// example 2
if(a === 'undefined'){	a = 100;}

// example 3
if(typeof a === 'undefined'){a = 100;}
console.log(a); // 100