访问嵌套对象的属性

Question

I'm new to Javascript, and currently learning it on codecademy.com

Here's the part I don't understand:

var friends = {};

friends.bill={};
friends.steve={};

friends.bill={
    firstName:"Bill",
    lastName:"Will",
    number:4164567889,
    address: ['298 Timberbank blvd', 'scarborough', 'ontario']
};

friends.steve={
    firstName:"Steve",
    lastName:"Evan",
    number:4161233333,
    address: ['111 Timberbank blvd', 'scarborough', 'ontario']


for ( var keys in friends){
    console.log(keys);                      // 1. returns 2 objects of friends(bill, steve)
    console.log(keys.firstName);            // 2. why is this wrong?
    console.log(friends[keys].firstName);   // 3. why the properties has to be accessed in this way?
}

so far the only thing I can assume is the "for..in" loop returns an array of the objects in "friends". that's why it has to be accessed by using [] notation. Correct me if I was wrong. Thank you

Answer 1

The for...in loop iterates over the keys of an Object.

So keys is either the string "bill" or "steve", not your friends objects. So keys.firstName evaluates to "bill".firstName and "steve".firstName , which of course is meaningless.

To get the object for the key "bill" in your friends object you can use either friends.bill or you can use friends['bill'] . Bot expressions are equal, but the later allows you to use a variable instead of a string known at compile time. Thus you can use keys instead of 'bill' like this: friends[keys] . Then this will actually be your object in friends.