Proxy mutable state with notifications and transformations.

MVar.js

/*
MVar is a facility for data flow among components and APIs.
*/
const assertFunction = (f, msg) => {
  if (typeof f !== 'function')
    throw new Error('Function required: ' + msg);
};
/**
 * Generically produces a new MVar chained after the given MVar.
 * @param m The given MVar.
 * @param f The subscribing function.
 * @returns {*} An MVar object.
 * @private
 */
const _after = (m, f) => {
  assertFunction(f, '');
  const n = {
    take: m.take,
    put: v => {
      m.put(v);
      f(v);
      return n;
    },
    before: f => _before(n, f),
    after: f => _after(n, f)
  };
  return n;
};
/**
 * Generically produces a new MVar chained before the given MVar.
 * @param m The given MVar.
 * @param f The transforming function.
 * @returns {*} An MVar object.
 * @private
 */
const _before = (m, f) => {
  assertFunction(f);
  const n = {
    take: m.take,
    put: v => {
      m.put(f(v));
      return n;
    },
    before: f => _before(n, f),
    after: f => _after(n, f)
  };
  return n;
};
/**
 * Creates an object that encapsulates a mutable value.
 * This is used to hook data update notifications, transform data from other sources, and proxy
 * data collection to other components, typically down the hierarchy.
 * Undefined values are forbidden.
 */
export const MVar = value => {
  if (value === undefined)
    throw new Error('Undefined values are forbidden.');
  const m = {
    take: () => value,
    put: v => {
      if (v === undefined)
        throw new Error('Undefined values are forbidden.');
      value = v;
      return m;
    },
    /**
     * Create a new MVar that executes function f after the value is set.
     * Typically, f redraws a component.
     * @param f Takes the incoming value.
     */
    after: f => _after(m, f),
    /**
     * Creates a new MVar that maps new values through f before setting.
     * Useful to coerce data, like ids from select options.
     * @param f
     */
    before: f => _before(m, f),
  };
  return m;
};