JS中undefined和null

JS中什么时候会出现undefined

1. 创建变量不赋值

let a;
console.log(a); //undefined

2. 空对象取属性值

let a = {};
console.log(a.name); //undefined

3. 调用有参数的函数,未给形参赋值

fn(a, b) {
	console.log(a, b); //undefined undefined
}
fn();

JS什么时候会出现null

1. 将 变量/对象 定义为null

 var a = {};// a = ''
 a = null;
 console.log(a);// null

2. getElementById获取元素，没有对应的ID

let a= document.getElementById("xxxId");
console.log(a); //null

3. 正则匹配时,未匹配到

let reg=/\d/
console.log( reg.exec("zzz"));//null