for i in range(a, b, c) is not translated to for (;;) loop

[faerot]

If range has extra arguments, it treats for loop like a loop over regular sequence.
It should be fairly trivial to make
for i in range(begin, end, step)
to translate into
for (i=begin; i < end; i += step)

[AlexECX]

Just tried it and it's working, for i in range(0, 5, 2) gives for (var i = 0; i < 5; i += 2).

[faerot]

Maybe it is working for constants, try using variables as arguments to see the bug.

[AlexECX]

After looking at the js source, my guess would be that the transpiler cannot know at compile time whether or not var step(edited) is positive or negative, so it can't decide which of i < end; and i > end; is the right code. This is taken care of at runtime by the range function.

[faerot]

maybe using != instead of < or > would solve the issue when step is not given or constant 1 or -1? using range() is a major overhead since it generates array every time. Don't want to resort to using while as it hurts readability.

[AlexECX]

Based on the range() function implementation, perhaps this could do:

for (var i = begin; step > 0 ? i < end : i > end; i += step){
    //stuff
}

It's about <1.5x slower than directly writing it in javascript, but orders of magnitude faster than the current implementation.

[faerot]

indeed, looks like a great solution for what it's worth

[faerot]

Another gotcha in the for loop:
in python the loop for i in range(10): assigns value to i each iteration so in the iteration you can modify it without fear. In transcrypt if it is unrolled into for (i = 0; i < 10; ++i) this will break the code if you will try to modify i within iteration.

Suggested solution - use temporary variable for looping and assign it to i each iteration

[AlexECX]

for i in range(0,2):
    i = i + 2

Already gives

for (var i = 0; i < 2; i++) {
    var i = i + 2;
}