match.js

/*
source: https://codeburst.io/alternative-to-javascripts-switch-statement-with-a-functional-twist-3f572787ba1c

the match() function is an alternative to a switch statement.
Benefits are:
The chain is an expression, unlike a switch statement.
Each case has its own scope.
The default case (.otherwise()) is mandatory.

    result = match(2)
      .on(x => x === 1, x => `${x} is one`)
      .on(x => x === 2, x => `${x} is two`)
      .on(x => x === 2, x => {
          console.log(`this shouldn't run`);
          return `${x} is two but we should never get here`;
      })
      .on(x => x === 3,x => `${x} is three`)
      .otherwise(x => `${x} is something else`);

result is "2 is two" AND console.log() was never called

alternatively pass a value instead of a function into the first
parameter of on()

    result = match(2)
      .on(2, x => `${x} is two`)
      .otherwise(x => `${x} is something else`);

*/

// source: https://stackoverflow.com/a/7356528
const isFunction = function isFunction(check) {
  return check && {}.toString.call(check) === "[object Function]";
};

const matched = x => ({
  on: () => matched(x),
  otherwise: () => x
});

// modified to optionally accept a single value instead of a function for first parameter
const match = x => ({
  on: (pred, fn) =>
    (isFunction(pred) ? pred(x) : pred === x) ? matched(fn(x)) : match(x),
  otherwise: fn => fn(x)
});

// optionally use this as a module, so in another file you can do
// const match = require(./match.js);
module.exports = match;