var f = {a, b, c};
It came with ES6 (ECMAScript 2015) and means exactly the same as:
var f = {a: a, b: b, c: c};
It is called Object Literal Property Value Shorthands (or simply property value shorthand, shorthand properties).
You can also combine shorthands with classical initialization:
var f = {a: 1, b, c};
For more information see Object initializer.