Question

所以我有两个for循环。在第一个循环中，消息正好说出了我想说的内容; Hello Herbert, Hello Peter。

但在第二个中它说; Hello undefined。

这两个代码块之间有什么区别，为什么第二个代码块返回undefined？

＆＃13;

let people = [
    {
        name: "Herbert",
        occupation: "Vildstaed",
        language: ["Finnish", "English", "German"]
    },
    {
        name: "Peter",
        occupation: "Skalnstead",
        language: ["German", "Livonian Dialect"]
    }
];

for(var i = 0; i < people.length; i++){
    alert("Hello " + people[i].name)
}

for(let person in people) {
    alert("Hello " + person.name)
}

＆＃13;

Answer 1

＆＃13;

let people = [
    {
        name: "Herbert",
        occupation: "Vildstaed",
        language: ["Finnish", "English", "German"]
    },
    {
        name: "Peter",
        occupation: "Skalnstead",
        language: ["German", "Livonian Dialect"]
    }
];

for(var i = 0; i < people.length; i++){
    alert("Hello " + people[i].name)
}

for(let person in people) {
    console.log(person);
    alert("Hello " + people[person].name)
}

＆＃13;

正如您所看到的，人们获得的值是数组的索引，因此您需要使用人员[person]

来访问它

Answer 2

For in循环将返回keys，而不是对象。因此，对于第二种情况，person不是您期望的那种情况。您只需显示person即可看到它。

＆＃13;

let people = [
    {
        name: "Herbert",
        occupation: "Vildstaed",
        language: ["Finnish", "English", "German"]
    },
    {
        name: "Peter",
        occupation: "Skalnstead",
        language: ["German", "Livonian Dialect"]
    }
];

for(let person in people) {
    alert("Hello " + person)
}

＆＃13;

要执行您想要的操作，您可以在for of版本中使用ES6循环，或者只需按键获取

＆＃13;

let people = [
    {
        name: "Herbert",
        occupation: "Vildstaed",
        language: ["Finnish", "English", "German"]
    },
    {
        name: "Peter",
        occupation: "Skalnstead",
        language: ["German", "Livonian Dialect"]
    }
];

for(let person of people) {
    alert("Hello " + person.name)
}

for(let person in people) {
    alert("Hello " + people[person].name)
}

＆＃13;

Answer 3

for(let person of people) {
   alert("Hello " + person.name)
}

Javascript未定义的值而不是值

3 个答案: