Intercept javascript value setter

Question

How can I listen to value set on a variable or object property without losing other listeners.

for example

var myModel;
myModel = 10;
var myListener1 = function(oldVal, newVal) {
    //do stuff
}
var myListener2 = function(oldVal, newVal) {
    //do other stuff
}

here I want myListener1 and myListener2 to be called whenever a value is set for variable myModel. Later in other functions might also want to add another listener to myModel on setter so it shouldn't override existing ones.

I know there should be a way to do it with Object.defineProperty(). Also it would be good to have a solution cross browser for IE8+.

Answer 1

For the following method, you would have to use an object with a property, but it works.

---

// this is very similar to using Object.watch()
// instead we attach multiple listeners
var myModel = (function () {
    var actualValue,
        interceptors = [];

    function callInterceptors(newValue) {
        for (var i = 0; i < interceptors.length; i += 1) {
            interceptors[i](newValue);
        }
    }

    return {
        get value() {
            // user never has access to the private variable "actualValue"
            // we can control what they get back from saying "myModel.value"
            return actualValue;
        },

        set value(newValue) {
            callInterceptors(newValue);
            actualValue = newValue;
        },

        listen : function (fn) {
            if (typeof fn === 'function') {
                interceptors.push(fn);
            }
        }
    };
}());

// add a listener
myModel.listen(function (passedValue) {
    alert('You tried to set myValue to ' + passedValue);
});

// add another listener
myModel.listen(function (passedValue) {
    alert('AAARRG! Why did you modify that value to ' + passedValue + '?!?!');
});

// our functions are called when we
// modify our value
myModel.value = 10;

---

jsFiddle example

Answer 2

only change the value of myModel through a function so you can run the listener functions after/before the change.

var myModel = 10,
    myListener1 = function(oldVal, newVal) {
      // do stuff
    },
    myListener2 = function(oldVal, newVal) {
     // do other stuff
    },
    changeMyModel = function (value) {
      myListener1(myModel, value);
      myListener2(myModel, value);
      myModel = value;
      // do more stuff 
    };

   // changeMyModel(99) === myModel; true

OR

   var change = function (value) {
      myListener1(myModel, value);
      myListener2(myModel, value);
      return value;          
   };
   // myModel = change(45);