Question

I’m in the process of writing a simple prototype in Node.js with some helper methods that I’ll probably need in objects using that prototype. One method that I’d like to have is an implementation of jQuery’s .each(). I’ve looked at jQuery’s implementation in their development release, and tried to emulate it in my simplified version here.

// Loop through the object using a callback
BaseProto.prototype.each = function (cb, args) {
    var obj = this.get(),   // Get our object as a plain object
        prop;

    /** Code to make sure the the args passed are actually an array **/

    if (typeof cb === "function") {
        // For each property in our object
        for (prop in obj) {
            // Apply the callback to the object's property and pass
            // the index, the property, and whatever else as arguments
            if (cb.apply(obj[prop], [ prop, obj[prop] ].concat(args)) === false) {
                // Get out if the function returns false
                break;
            }
        }
    }

    // Reset our object with the new one
    return this.reset(obj);
};

The problem is that while the callback is definitely being fired, it doesn’t have any effect on the object’s property. No matter what I do inside the callback, the changes stay inside the callback’s scope.

Here is an example of a simple callback that I’ve been testing with.

var BaseProtoTestObj = new BaseProto();

/** Set some properties to BaseProtoTestObj **/

function cb1 ( key, val ) {
    var prop;

    key = key.toString() + " Callbacked";
    val = val.toString() + " Callbacked";

    for (prop in this) {
        this[prop] = this[prop].toString() + " Callbacked";
    }
}

// Doesn't have any effect on BaseProtoTestObj
BaseProtoTestObj.each(cb1);

I can see that there is a lot more going on in jQuery’s .each(), but from what I can gather it’s for optimization and the ability to iterate over arrays as well as objects.

In the end, my question is simple. What is it that jQuery is doing to affect the properties, that I’m not in my .each()?

---

Edit

I suppose another question to ask would be if my logic is fundamentally wrong, and you can’t modify an object’s properties this way.

Answer 1

You don’t need a custom method:

for(var prop in object) {
   var value = object[prop];
   // do something with value and/or prop
}

Although if you really needed .each(), you could do something like this:

Object.prototype.each = function(cb) {
    for(var propName in this) {
        cb(propName, this[propName]);
    }
}

var foo = { prop: 'value', prop2: 'value2' };
foo.each(function(key,value) {
    // do something here
});    

Since you need to modify the actual values of the properties, try this:

Object.prototype.mutate = function(cb) {
    for(var propName in this) {
        this[propName] = cb(propName, this[propName]);
    }
}

var obj = {
    a: 'foo',
    b: 'bar',
    c: 'baz'
};

obj.mutate(function(propName, propValue) {
    return propName + '-' + propValue;
});

/*
obj will now be:

var obj = {
    a: 'a-foo',
    b: 'b-bar',
    c: 'c-baz'
};
*/