Ruby's ||= (or equals) in JavaScript?

Both are absolutely correct, but if you are looking for something that works like ||= in ruby. The first method which is variable = variable || {} is the one you are looking for :)

You can use the logical OR operator || which evaluates its right operand if lVal is a falsy value.

Falsy values include e.g null, false, 0, "", undefined, NaN

x = x || 1

If you're working with objects, you can use destructuring (since ES6) like so:

({ myLib: window.myLib = {} } = window);

...but you don't gain anything over the accepted answer except confusion.