forinloop.md

The for in Loop

Just like the in operator, the for in loop traverses the prototype chain when iterating over the properties of an object.

Note: The for in loop will not iterate over any properties that have their enumerable attribute set to false; for example, the length property of an array.

// Poisoning Object.prototype
Object.prototype.bar = 1;

var foo = {moo: 2};
for(var i in foo) {
    console.log(i); // prints both bar and moo
}

Since it is not possible to change the behavior of the for in loop itself, it is necessary to filter out the unwanted properties inside the loop body. In ECMAScript 3 and older, this is done using the hasOwnProperty method of Object.prototype.

Since ECMAScript 5, Object.defineProperty can be used with enumerable set to false to add properties to objects (including Object) without these properties being enumerated. In this case it is reasonable to assume in application code that any enumerable properties have been added for a reason and to omit hasOwnProperty, since it makes code more verbose and less readable. In library code hasOwnProperty should still be used since assumptions cannot be made about which enumerable properties might reside on the prototype chain.

Note: Since for in always traverses the complete prototype chain, it will get slower with each additional layer of inheritance added to an object.

Using hasOwnProperty for Filtering

// still the foo from above
for(var i in foo) {
    if (foo.hasOwnProperty(i)) {
        console.log(i);
    }
}

This version is the only correct one to use with older versions of ECMAScript. Due to the use of hasOwnProperty, it will only print out moo. When hasOwnProperty is left out, the code is prone to errors in cases where the native prototypes - e.g. Object.prototype - have been extended.

In newer versions of ECMAScript, non-enumerable properties can be defined with Object.defineProperty, reducing the risk of iterating over properties without using hasOwnProperty. Nonetheless, care must be taken when using older libraries like Prototype, which does not yet take advantage of new ECMAScript features. When this framework is included, for in loops that do not use hasOwnProperty are guaranteed to break.

In Conclusion

It is recommended to always use hasOwnProperty in ECMAScript 3 or lower, as well as in library code. Assumptions should never be made in these environments about whether the native prototypes have been extended or not. Since ECMAScript 5, Object.defineProperty makes it possible to define non-enumerable properties and to omit hasOwnProperty in application code.