functional arguing

The arguments variable is a hidden parameter in javascript functions that contains all the arguments for the invocation in a pseudo-array (ie: it does not have array functions other than length and indexing).

instance.headline = function (h) {
  if (!arguments.length) return headline;
  headline = h;
  return instance;
};

Even though the function specifically calls for an argument (h), it can still be called without one. This construction has a ruby equivalent, attr_accessor :headline, which behaves as a getter and a setter, depending on the arguments provided (none, or headline = new_value).

This “variable-parameter” function design is what enables method chaining. When the function is called as a getter it returns a string, but when it is called as a setter it returns the invoking object, passing itself along to the next method:

var widget = SimpleWidget({ color: "#6439ed" })
                .headline("SimpleWidget")
                .description("This is a simple widget")
                .render();

Resources

Data Visualization with D3.js Cookbook (affiliate link)