Today's violation of the Principle of Least Surprise

[wrog]

  var answer = [];

  for (word in ['this','language','can','bite','me']) {
     answer.push(word);
  }

Just to make it easy, I'll tell you that the language in question is Javascript, [...] is an array constructor, and .push appends its arguments to the end of the list/array. The answer, of course, is...

  [0,1,2,3,4]

See, Javascript doesn't actually have arrays, Javascript has objects with numbered properties. And since foo.['whatever'] is defined to be foo.whatever with numbers and strings automagically converting to each other as needed, you'll never, ever notice the difference.

Unless, of course, you try to use for, which iterates through property names, not property values.

I suppose, to be completely fair, I should note that in Perl, some idiot could conceivably do

  for my $word (['this','language','can','bite','me']) {
     push @answer, $word;
  }

and get a list of length 1 for their trouble. Oh, well...

Comments

[dougo.livejournal.com]

I was surprised that "push" in Javascript appends rather than prepends. Push is for stacks, not queues.

[wrog]

they're clearly following what Perl does, since the Array prototype also defines .shift() and .pop()

and it's still a stack; it just grows the other way (except since these aren't really arrays in any kind of physicalmemory sense, there isn't really any "way" here, but anyway...)

[dougo.livejournal.com]

Well, it's a stack in that .pop() pops off the end, but if you iterate over the stack it would go from bottom to top, which is weird. (Unlike Lisp push/pop.)