CODEWITHSUNDEEP
coffeescript
Tonči D.
Tonči D.

Reputation: 464

Array element presence check quirks in CoffeeScript

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?

Upvotes: 0

Views: 47

Answers (1)

Tonči D.
Tonči D.

Reputation: 464

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

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

Upvotes: 1

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