Question

myObj.FirstName = 'Phillip',
myObj.LastName = 'Senn';
for (var X in myObj) // FirstName LastName
for each (var X in myObj) // Phillip Senn

Q: Mentally, how do you read these two statements?

Answer 1

The first one (for ( in )) is reading property names from the object.

So you may read it as for each property in myObj, assign it to x.

The second one (for each ( in )) is reading values of the properties in the object.

This one may be read as for each property’s value in myObj, assign it to x.

Note that for each has limited browser support.

Also note that if extra properties are appearing in for ( in ), it is because it will look up the prototype chain for extra enumerable properties (and someone may have augmented Object, for example).

You can mitigate this with…

for (var x in myObj) {
   if ( ! myObj.hasOwnProperty(x)) {
       continue;
   }
   // Now you are sure the property is of `myObj`
}

jsFiddle.