Bracket Notation for JavaScript Objects

The bracket notation way of setting JavaScript objects is a funny thing since it's really easy to forget about when you think of objects in terms of JSON, but the square bracket notation can be super useful in certain situations, particularly when you want to set an object's key to the value of a JavaScript variable.

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A Real-World Example

I had a list of users in my trusty Firebase database, and I was trying to implement a "following" property on each user object. This following node would actually contain a list of key-value pairs where the value is 1 and the key is the unique id (generated by Firebase when the user first signs up) of the user you want to follow. I was having trouble creating a new object with one key/value property whose key was set dynamically (I was having trouble, that is, until I remembered about the bracket notation).

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Objects

​Ok, so here's the situation. I have a user logged in through Firebase, and now I have his or her unique identifier (uid) saved into the variable loggedInUser.

const currentProfileUser = profileDisplay.uid;
const loggedInUser = firebase.auth().currentUser.uid

Suppose, now, that I want to create a new object with a parameter whose key is equal to the value of the variable named loggedInUser. Naively creating the object like this:

var obj = {
    loggedInUser:1
};

just results in the new object having a key of the exact text, "loggedInUser". This is not what we wanted. We wanted the VALUE of loggedInUser to be the key; in other words the actual uid. 

Luckily, we can create an object and then use bracket notation to set a parameter. It would look something like this:

var obj = {};
obj[loggedInUser] = 1;

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The Awesomeness of Firebase

If you're curious, here's the JavaScript code for setting this object in Firebase. Remember, the Firebase database is just a super fast JSON object in the cloud so you can easily set more complex blocks of data that are just JavaScript objects.

var myRef = firebase.database()
    .ref('/users/' + self.auth.currentUser.uid + '/following/');

myRef.set(obj, function (error) {
    console.log('ok ' + error);
});