Easy pattern matching in JavaScript

pattern-matching.js

'use strict';

const buildMatcher = (cases) => (actual) =>
  cases
    .find((cse) => cse.matches(actual))
    .result(actual);

const buildNextOn = (previousCases) => {
  var build = (matchesPred, result) => {
    const cases = previousCases.concat(
      [
        {
          matches: matchesPred,
          result: result
        }
      ]
    );

    return {
      match: buildMatcher(cases),
      on: buildNextOn(cases)
    };
  };

  return {
    value: (v, result) => build(
      (actual) => actual === v,
      result
    ),
    anything: (result) => build(
      (_) => true,
      result
    ),
    object: {
      withProperties: (properties, result) => build(
        (value) => properties.every((prop) => prop in value),
          result
      )
    }
  };
};

const on = buildNextOn([]);

console.log(
  on.value(35, () => 27).
  on.value(42, () => 35).
  match(42)
);
// => 35

var fn = on.value(35, () => 27).on.value(42, () => 35).match;

console.log(fn(35));
// => 27

var factorial =
  on.value(1, () => 1).
  on.anything((n) => n * factorial(n - 1)).
  match;

console.log(factorial(4));
// => 24

var objectMatcher =
  on.object.withProperties(['a', 'b'], (obj) => obj.b).
  on.object.withProperties(['a', 'c'], (obj) => obj.c).
  on.anything((_) => { return { not: 'matched' } }).
  match;

console.log(objectMatcher({ a: 'foo', c: 'bar' }));
// => 'bar'

console.log(objectMatcher({}));
// => { not: 'matched' }