Watcher

watcher.js

function Watcher(expression, handler, deep) {
  this._expression = expression;
  this._handler = handler;
  this._deep = deep;
  this._old = undefined;
}

Watcher.prototype = {
  digest: function(){
    var _new = this.expression();
    var changed = !this.equals(this._old, _new);
    if (changed) this._handler(_new, _old);
    this._old = _new;
  },

  equals: function(a, b) {
    return this._deep ? _.isEqual(a, b) : a === b;
  }
}