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observe-function.js

//javascript:
function observeFn(obj, fnName, before, after) {
  var originalFn = obj[fnName],
    noop = function () {};
  before = typeof before === 'function' ? before : noop;
  after = typeof after === 'function' ? after : noop;
  obj[fnName] = function () {
    var args = [].slice.call(arguments);
    before({
      args: args,
      context: this
    });
    var retval = originalFn.apply(this, args);
    after({
      retval: retval,
      args: args,
      context: this
    });
  };
}

observe-property.js

//javscript:
function observeProp(obj, prop, fn) {
  var val = obj[prop];
  Object.defineProperty(obj, prop, {
    configurable: true,
    enumerable: true,
    get: function () {
      return val;
    },
    set: function (_val) {
      val = _val;
      fn();
    }
  });
}