Very slow console.log on large sparse arrays compared to Node and Deno

[kevgeoleo]

What version of Bun is running?

1.3.11-canary.1+9e93bfa1b

What platform is your computer?

Linux 5.10.0-12-amd64 x86_64 x86_64

What steps can reproduce the bug?

Hi,

I would like to report a behavior in Bun that can be reproduced using the below snippet

console.time("long-empty-array");
console.log(new Array(167772160 * 10));
console.timeEnd("long-empty-array");

What is the expected behavior?

Node takes just 9ms

root@KContainer:~/14656# node long_empty_array_log.mjs
[ <1677721600 empty items> ]
long-empty-array: 9.408ms

Deno takes just 0.5ms

root@KContainer:~/14656# deno run -A long_empty_array_log.mjs
[ <1677721600 empty items> ]
long-empty-array: 0.578ms

What do you see instead?

Bun takes 16seconds which is abnormally slow

root@KContainer:~/14656# bun run long_empty_array_log.mjs
[
  1677721600 x empty items
]
[16.15s] long-empty-array

Additional information

No response

[robobun]

✅ Reproduced — new Array(1_677_721_600) took ~16s because the console formatter looped i=0..len calling getDirectIndex for every hole.

Fix: inspect the JSArray butterfly directly to skip runs of holes in O(1). new Array(1_677_721_600) now prints in under 1ms, matching Node.

PR → #29178