Length of empty array in javascript

Question

so i'm trying to do one of the most basic things, and i am wondering what's wrong with my approach.

i have an object containing rooms

{ rooms: 
   { ijxeczyu: 
      { Id: 'ijxeczyu',
        teacherName: 'testteacher',
        teacherId: '/#Ikosyfr1NtDcpKbaAAAA',
        clients: [] } },
  getRooms: [Function],
  addRoom: [Function] }

now i want to render their info to a table in jade. for that to work, i want to extract the important stuff and push it to an array

function makeArray(obj) {

  var roomArray = [];

  for (var room in obj) {
    roomArray.push({
      roomId  : room.Id,
      teacher : room.teacherName,
      clients : room.clients.length || 0
    })
  }
  return roomArray;
}

pretty easy. but i can't get that empty clients array to work. TypeError: Cannot read property 'length' of undefined isn't that exactly what the || 0part should catch?

why doesn't this work? how can i read the length of an array or 0 if there are no entries?

Presuming you mean that room.clients is undefined, then the || 0 is too late. Instead, you can do: (room.clients || []).length — Nick White
– Nick White, Commented Jan 27, 2016 at 22:39

Mike Cluck · Accepted Answer · 2016-01-27 22:41:02Z

4

room.clients is undefined which is why you get that error. The reason it's undefined is because room is a string.

A for..in loop iterates over the keys of an object. This means that room === "ijxeczyu", assuming you're passing in your rooms object. To get access to the object, you'll have to access the object at that key.

for (var name in obj) {
  var room = obj[name];
  ...
}

answered Jan 27, 2016 at 22:41

Mike Cluck

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3 Comments

user3787706 Over a year ago

aaaaahhhh ok. now it makes sense. so basically, a for..in loop is pretty much no advantage over a standard for loop?

Mike Cluck Over a year ago

@user3787706 The advantage is that you can use it to iterate over the keys of an object. A standard for loop is for iterating over arrays. for (var key in obj) vs. for (var i = 0; i < array.length; i++). You could also get the keys using Object.keys(obj) then use a standard for loop over those.

Barmar Over a year ago

Ecmascript 7 adds for value of object, which iterates over the values instead of the keys.

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