Adding function to Object prototype causes function to show up in all 'for X in OBJ' loops

Question

So, here's some sample javascript code:

Object.prototype.simpleFunction = function () {
    return true;
}
var tempObject = {};
for (var temp in tempObject) {
    console.log(temp);
}

Note that if you execute this, you'll get 'simpleFunction' output from the console.log commands in Google Chrome. (I'm using 19.0.1084.46m .)

However, the wide variety of related Object functions are not passed to the console.log.

How can I add functions onto the Object prototype without them showing up in my 'for property in object' loops?

Edit: I should have mentioned that the last thing I wanted was to throw another 'if' statement in there, as it'd mean I'd need to add it to ALL for loops. :(

Answer 1

Which is why you should always check hasOwnProperty:

for (var temp in tempObject) {
    if (Object.prototype.hasOwnProperty(tempObject, temp)) {
        console.log(temp);
    }
}

Crockford advocates using Object.prototype.hasOwnProperty instead of tempObject.hasOwnProperty, just in case you override hasOwnProperty in your object.

---

In ES5, you can set it to not be enumerable:

Object.defineProperty(Object.prototype, 'simpleFunction', {
    value: function() {
        return true;
    },
    enumerable: false, // this is actually the default
});

Alternatively (in ES5), you can use Object.keys() to only get the object's own keys:

Object.keys(tempObject).forEach(function(key) {
    console.log(key);
});