声明而没有赋值的变量是undefined
没有返回值的函数返回的是undefined
var a;//undefined console.log(a);//undefined console.log(typeof a);//undefined console.log(typeof a == undefined)//false console.log(typeof a == "undefined")//true console.log(typeof a === undefined)//false console.log(typeof a === "undefined")//true console.log(a==null);//true console.log(a==undefined);//true console.log(a===null);//false console.log(a===undefined);//true console.log("*******************"); var b = null;//null console.log(b);//null console.log(typeof b);//object console.log(typeof b == undefined)//false console.log(typeof b == "undefined")//false console.log(typeof b === undefined)//false console.log(typeof b === "undefined")//false console.log(b==null);//true console.log(b==undefined);//true console.log(b===null);//true console.log(b===undefined);//false