obj.name ==> obj["name"]  底层的自动转化,所以直接写 obj["name"] 效率会高一些

var obj = {
    name: "zhangsan",
    age: 12,
    sex: "male"            
}

console.log(obj.name); // "zhangsan"
console.log(obj["name"]); // "zhangsan"
console.log(obj[name]); // 会报错: name is not defined

常见错误: 变量字符串化

var obj = {
    name: "zhangsan",
    age: 12,
    sex: "male"
}
for (var prop in obj) {
    console.log(obj.prop); // undefined * 3  : prop不再是变量 转化为obj["prop"]
}