javascript 容易误解的地方

(1)使用for…in遍历对象时,不仅会遍历对象中的属性和方法,同时也会遍历类的原型中的属性和方法

var Person=function(){
  this.username="huangweii";
  this.age=27;
  this.sayHello=function()
  {
    console.log("username is :"+this.username);
  }
}
Person.prototype.sex=‘man‘;
Person.prototype.eat=function(){
  console.log(‘eat...‘);
}
var person=new Person();
person.sayHello();
console.log(‘------------------------‘);
for(var i in person){
  console.log("key:\t"+i);
}

运行结果:

(2)

alert(name123);

运行结果:
报错:ReferenceError: name123 is not defined

alert(name123);
//var name123=”whuang”;
但是:

alert(name123);
var name123="whuang";

运行结果:undefined

(3)

function sayHello(username,age)
{
  arguments[1]=23;
  console.log(‘my name is ‘+username+" , age is "+age);
}

sayHello(‘whuang‘,24);

预期结果:my name is whuang , age is 24
运行结果:
my name is whuang , age is 23

(4)声明但是不赋值就不会改变原来的值

var username333=‘whuang‘;
var username333;
console.log(username333);

运行结果:
whuang

经验:当我们不确定变量是否被声明,但是又需要调用时,我们可以声明一下,防止报错.
因为声明但是不赋值就不会改变原来的值