Array element presence check quirks in CoffeeScript

Question

When checking element presence in inline arrays via the in keyword, CoffeeScript ignores indexOf and rather does that many equality checks.

However, when referring to an array via a variable, it calls indexOf as expected.

Input

foo = 1

# -----------
foo in [1, 2, 3, 4, 5]

# -----------
bar = [1, 2, 3, 4, 5]
foo in bar

Output

var bar, foo,
  indexOf = [].indexOf;

foo = 1;

// -----------
foo === 1 || foo === 2 || foo === 3 || foo === 4 || foo === 5;

// -----------
bar = [1, 2, 3, 4, 5];

indexOf.call(bar, foo) >= 0;

Demo code: Link

I find this intriguing. Any ideas why?

Interesting quirk. Might be intentional optimization where it's assumed if the array is hardcoded, it's not very long. Might be a bug. I'd recommend asking this question in the repository: github.com/jashkenas/coffeescript/labels/question — caffeinated.tech
– caffeinated.tech, Commented Jan 14, 2022 at 11:52
Done - thanks for the suggestion: github.com/jashkenas/coffeescript/issues/5392 — Tonči D.
– Tonči D., Commented Jan 17, 2022 at 9:59

Tonči D. · Accepted Answer · 2022-01-19 08:38:18Z

1

Apparently, this was an intended optimization to spare an array allocation.

From: https://github.com/jashkenas/coffeescript/issues/5392

answered Jan 19, 2022 at 8:38

Tonči D.

4641 gold badge4 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Array element presence check quirks in CoffeeScript

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest