My feeling is that even the compilers written in CS101 will optimize this. I'm guessing that Google tested their code with V8, performance was fine, and they thought nothing of it.

I just did a benchmark with node.js. I made a 50000000 element array, and timed how long each way took.

Trial one:

    for( var i = 0; i < array.length; i++ ) { array[i]++ }

That took, on average, 0.93001866 seconds.

Trial two:

    for( var i = 0; i < len; i++ ) { array[i]++ }

That took, on average, 0.809920 seconds.

A lot of stressing-out over what ends up being a rounding error.