myHook.js

export default class Watcher {
  constructor() {
    this._mem = [];
  }

  _compareFunction(val1, val2) {
    return Object.is(val1, val2);
  }

  setCallbacks(callbacks) {
    callbacks.forEach((cb, i) => {
      this._mem[i] = cb(this._mem[i]);
    });
  }

  whenChange(variables, callback, customCompare) {
    return prevVariables => {
      if (prevVariables === undefined) {
        callback(...variables);
        return variables;
      }

      const nothingChanges = variables.every(
        (currVar, i) => typeof customCompare === 'function'
          ? customCompare(currVar, prevVariables[i])
          : this._compareFunction(currVar, prevVariables[i])
      );

      if (nothingChanges) return variables;
      callback(...variables);
      return variables;
    };
  }

}

test('whenChange() with [props] shoot callback only when props change', t => {
  let counter = 0;

  let props = 'foo';
  watcher.setCallbacks([
    watcher.whenChange([props], () => counter = counter + 1)
  ]);

  watcher.setCallbacks([
    watcher.whenChange([props], () => counter = counter + 1) // Must not call
  ]);

  props = 'bar';
  watcher.setCallbacks([
    watcher.whenChange([props], () => counter = counter + 1)
  ]);
  
  t.is(counter, 2);
});