I highly doubt js engines would compile a string down to a linked list. But you're right they might compile it to a circular buffer or deque which can have O(1) prepends.

Quick googling shows that this optimization might exist but only for firefox and only if you use "unshift": https://lannonbr.com/blog/2020-01-27-shift-optimizations https://jandemooij.nl/blog/2017/12/06/some-spidermonkey-opti...

But it's very unlikely that the jit can optimize `str = ch + str;`