Stop to use Math.floor
The first one is about usage of Math.floor. It is probably only my opinion, but it seems that Math.floor is used always to perform the same task:
var centerWidth = Math.floor((something + someelse - someother) / 2);
The point is that at the end of a Math.floor, you will often find that division by 2.
There is a truly simple way to write less, and to obtain best performances as well, it is the right side bitwise operator, that for this purpose is nearly perfect.
var centerWidth = (something + someelse - someother) >> 1;
That's it! If you compare above examples you will note that second one is about 2X faster than Math.floor.
for(var i = 0, t1 = new Date; i < 50000; i++)
Math.floor(i / 2);
t1 = new Date - t1;
for(var i = 0, t2 = new Date; i < 50000; i++)
i >> 1;
t2 = new Date - t2;
A tricky String.indexOf
Another common piece of code you can find wherever, is this one:
if(myWord.indexOf(myChar) >= 0) ...
if(-1 < myWord.indexOf(myChar)) ...
if(myWord.indexOf(myChar) !== -1) ...
There is an operator that inverts the number, adding +1, and that is perfect every time we have a function that could return 0, more than zero, or -1 where there's no match.
Above example converts every number from 0 to N into -1 to -(N+1)
The latter one is probably not known as the first one, and its execution time is about the same, but you write less, and you can recognize instantly if that method or function respect the integer -1 to +N return value.
The implicit cast when you use an if or a ternary operator, is the one that makes that check fast enough, but not faster than common way as is the first suggested trick.
P.S. these tricks could be used with many other program languages ;)