Square Brackets Javascript Object Key

It's the new ES2015 (the EcmaScript spec formally known as ES6) computed property name syntax. It's a shorthand for the someObject[someKey] assignment that you know from ES3/5:

var a = "b"
var c = {[a]: "d"}

is syntactic sugar for:

var a = "b"
var c = {}
c[a] = "d"

const animalSounds = {cat: 'meow', dog: 'bark'};

const animal = 'lion';

const sound = 'roar';

{...animalSounds, [animal]: sound};

The result will be

{cat: 'meow', dog: 'bark', lion: 'roar'};

Really the use of [] gives an excellent way to use actual value of variable as key/property while creating JavaScript objects.

I'm pretty much statisfied with the above answer and I appreciate it as it allowed me to write this with a little example.

I've executed the code line by line on Node REPL (Node shell).

> var key = "fullName";     // Assignment
undefined
>
> var obj = {key: "Rishikesh Agrawani"}    // Here key's value will not be used
undefined
> obj     // Inappropriate, which we don't want
{ key: 'Rishikesh Agrawani' }
>
> // Let's fix
undefined
> var obj2 = {[key]: "Rishikesh Agrawani"}
undefined
> obj2
{ fullName: 'Rishikesh Agrawani' }
>