Ruby has a conditional assignment operator: ||=. JavaScript doesn’t have such an operator. Common use cases for conditional assignment that JavaScript developers use are lazily defining a namespace and defaulting function arguments.

Since JavaScript has no built in conditional assignment operator, the language leaves conditional assignment as a stylistic choice for JavaScript developers. The following is the style I like, which I learned from Jeremy Ashkenas.

• Defining a namespace:

    var Namespace;
    Namespace || (Namepace = {});

• Function arguments:

    Namespace.Model = {};
  
    _.extend(Namespace.Model, {
      set: function(attrs, options) {
        options || (options = {});
  
        if (options.logger) {
          return options.logger.log("Hello. Yes, This is log.")
        }
      }
    });