for in loop behaving strangely in javascript

Question

var friends=['2','3'];
console.log(friends);
var rooms=[];
for(var friend in friends){
var room = 'userid'+friend+'friends';
console.log(room);
rooms.push(room);
}
console.log(rooms);    

this outputs

[ '2', '3' ]
 userid0friends
 userid1friends
 [ 'userid0friends', 'userid1friends' ]

3 is neglected entirely, and it behaves even more strangely on my node.js server

Answer 1

friend here is the index of your array, not the value at the index

var room = 'userid'+friends[friend]+'friends';

Plus when looping through an array I don't recommend to use for..in loops, you can use Array.prototype.map or plain for loop with the length of your array

Array map example:

var friends=['2','3'];
console.log(friends);
var rooms= friends.map(function (friend, index) {
    return 'userid' + friend + 'friends';
});
console.log(rooms);

Plain for loop:

var friends=['2','3'];
console.log(friends);
var rooms=[];
for (var i = 0, l = friends.length; i < l; ++i) {
    var room = 'userid' + friends[i] + 'friends';
    console.log(room);
    rooms.push(room);
}
console.log(rooms);

Answer 2

The for in structure is used to loop the object, for key in object, key is the property name of the object, when used for an array, key will be the array index.

You could use .map to get your result:

var friends=['2','3'];

var rooms = friends.map(function(e) {
  return 'userid'+e+'friends';
});

Answer 3

A for in loop is used to enumerate objects. You are trying to iterate over an array. A better technique for iterating would be a regular for loop or the Array.forEach() method.

In a for in loop, the variable contains the key, not the value. To get the value, use the key:

for(var i in friends){
    var friend = friends[i];
    ...
}

But, again, this enumerates an object, including any properties besides the array elements. This means that if any code adds a method to Array.prototype, that method will also be enumerated. Using .forEach() is better:

friends.forEach(function (friend, i) {
    ...
});

Better yet, since you are trying to create a new Array based on the contents of another, use Array.map(). That is exactly what it's for.

var rooms = friends.map(function (friend, i) {
    return 'userid'+friend+'friends';
});