Introduction

Many D3 plugins are intended to be invoked by selection.call, meaning that they produce a function that can be configured and then passed to selection.call. For example, d3-axis provides a number of constructors (axisLeft, axisTop, etc.) that all produce an axis, which is a function. It is also an object with methods for modifying the scale and ticks.

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)$/;