Javascript undefined value instead of value

Question

So I have these two for loops. In the first loop the message says exactly what I want it to say; Hello Herbert, Hello Peter.

But in the second one it says; Hello undefined.

What is the difference between these two blocks of code, and why does the second one returns undefined?

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)
}

marvel308 · Accepted Answer · 2017-08-20 09:42:14Z

2

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)
}

As you can see the values person is getting is the index of the array, hence you need to access it using people[person]

answered Aug 20, 2017 at 9:42

marvel308

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Comments

Suren Srapyan · Accepted Answer · 2017-08-20 09:43:51Z

For in loop will return to you the keys, not the objects. So for the second case you the person is not the one which you expect. You can see it by showing only the person.

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)
}

To do what you want you can use for of loop instead in ES6 version or just get by key

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)
}

syntaxe · Accepted Answer · 2017-08-20 09:45:20Z

0

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

answered Aug 20, 2017 at 9:45

syntaxe

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