List comprehensions are incredible. But they're just syntactic sugar for map() and filter() and lambdas.

Python's

  prev = [1,2,3,4]
  next = [x * 2 for x in old]

Is the same as javascript's

  prev = [1,2,3,4];
  next = prev.map(x => x * 2);

Python's

  next = [x for x in prev if x % 2 == 0]

is the same as javascript's

  next = prev.filter(x => x % 2 === 0);

And finally, python's

  next = [x * 2 for x in prev if x % 2 == 0]

is the same as

  next = prev.filter(x => x % 2 == 0).map(x => x * 2);

It could be nice to have a "mapIf" function that accepts a mapping and a predicate, but I think the extra verbosity makes the above code easier to understand.

  next = prev.mapIf(x => x * 2, x => x % 2 == 0);