On: Stanford's CS 101 course uses Javascript

[__rkaup__]

Are you saying im.getWidth() is doing some expensive calculation underneath, or that the cost of calling it is expensive in Javascript?

[__mark → __rkaup__]

I imagine it loops though the full width of the image every time it is called, which is every time the loop reaches the check.

Since storing it will require width-1 less calls to im.getWidth() it is much more efficient no matter how you slice it.