JavaScript Quickie: Setting a variable to undefined defines it (sorta)
Worked this out while answering a Stack Overflow question, and thought it was worth sharing.
Basically, assigning anything, including undefined to a variable or property will bring it into existence. This probably doesn’t matter all that often, but it can cause a sparse array to not be all that sparse.
arr = [42,69,,66];
arr.hasOwnProperty('0'); // returns true, because arr[0] == 42
arr.hasOwnProperty(''2'); // returns false, because that element doesn't exist
arr.hasOwnProperty('3'); // returns true
alert(arr[2]); // will alert 'undefined'
arr[2] = undefined;
alert(arr[2]); // still alerts 'undefined'
arr.hasOwnProperty('2'); // now returns true, as it now exists
delete arr[2];
arr.hasOwnProperty('2'); // returns false, because it no longer exists
The Point: Just because a variable or property is undefined, doesn’t mean it isn’t taking up some space. To be sure, you need to check and delete it if it exists.
