Square Brackets Javascript Object Key

Can anyone explain how the why/how the below method of assigning keys in JavaScript works?

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

return:

Object {b: "d"}

Answer

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"