Mixing bracket and dot notation confusion

JavaScript
1

Tell us what’s happening:
I do not understand why dot notation must be combined with bracket notation here. The user is a property of usersObj, and online is a property of user, so why can’t we access the value using either usersObj.user.online or usersObj[user][online]? Why does it have to be a combination? This seems rather arbitrary to me, so I must be missing something important here.

  **Your code so far**


function countOnline(usersObj) {
// Only change code below this line
let count = 0;
for (let user in usersObj) {
  if(usersObj[user].online == true) {
    console.log(usersObj[user].online); 
    count++;
  }
}
return count;
// Only change code above this line
}

  **Your browser information:**

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.85 Safari/537.36.

Challenge: Iterate Through the Keys of an Object with a for…in Statement

Link to the challenge:

2

They are different because user is a variable that contains a string that it an object property and “online” is an object property.

This:

usersObj[user].online

works because it will look for the property that matches the string in user and then access the property “online” off of that object.

This:

usersObj.user.online

would always try to access a property called “user”. If you have a property named that, it would work, but just for that one property.

This:

usersObj[user][online]

Won’t work because you don’t have a variable online. If you put:

const online = `online`;

before it, it would work.

This would actually work:

usersObj[user]['online']

because you are using bracket notation but are just passing it the string you want. It is functionally equivalent to:

usersObj[user].online

Does that help?

3

Makes perfect sense now! Thanks!

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4

And just to be clear…

Generally you will use dot notation. The two cases where you use bracket notation:

  1. The key/property name is stored in a variable.
  2. The key/property name is not a valid JS identifier, like if it has a space or a hyphen in it.
5

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