d3-compose/src/compose.js

Utility function that supports composition for `d3.call`s.

// Introduction
// ============
//
// Many D3 plugins are intended to be invoked by [`selection.call`][call],
// meaning that they produce a function that can be configured and then passed
// to `selection.call`.  For example, [d3-axis][d3-axis] provides a number of
// constructors (`axisLeft`, `axisTop`, etc.) that all produce an
// [`axis`][axis], which is a function.  It is also an object with methods for
// modifying the scale and ticks.
//
// [call]: https://github.com/d3/d3-selection/blob/master/README.md#selection_call "Selection#call (d3-selection)"
// [d3-axis]: https://github.com/d3/d3-axis "d3-axis"
// [axis]: https://github.com/d3/d3-axis#_axis "axis (d3-axis)"

// `compose` takes a list of functions, `fns`, intended to be passed to
// `selection.call` and returns a function that when passed to `selection.call`
// invokes each function in turn, from right to left, as if they were passed to
// `selection.call` one at a time.  The attributes of `fns` are merged into the
// returned object.
export default function compose(...fns) {
  // Construct a new function that when called enumerates over the functions in
  // the composition and applies each in turn.
  let f = function(...args) {
    for (let f of fns) {
      f.apply(this, args);
    }
  };

  // The new function is now assigned each of the attributes from the source
  // functions.
  for (let source of fns) {
    for (let k in source) {
      if (!nonEnumerableProps.test(k)) {
        f[k] = source[k];
      }
    }
  }

  // Reverse the order of the functions so that they are applied from right to
  // left.
  fns = fns.reverse();
  return f;
}

// These properties are not to be touched during the merge.
const nonEnumerableProps = /^(valueOf|isPrototypeOf|to(Locale)?String|propertyIsEnumerable|hasOwnProperty)$/;