The for in loop

Just like the in operator, the for in loop also 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 , this is done by using the hasOwnProperty method of Object.prototype.

Note: Since the 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. Due to the use of hasOwnPropery 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.

One widely used framework which does this is Prototype. When this framework is included, for in loops that do not use hasOwnProperty are guaranteed to break.

Best practices

It is recommended to always use hasOwnProperty. Never should any assumptions be made about the environment the code is running in, or whether the native prototypes have been extended or not.

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Intro	Objects	The prototype
hasOwnProperty	Functions	How this works
Closures and references	The arguments object	Scopes and namespaces
Constructors	Equality and comparisons	Arrays
The Array constructor	The for in loop	The typeof operator
The instanceof operator	Type casting	undefined and null
Reasons against eval	setTimeout and setInterval	Automatic semicolon insertion