Could you explain this change? I would expect allocating an object to be bad, since then it needs to be garbage collected.

Would this have the same effect?

if (i < val.length) {
  const subKey = `${key}[${i}]`;
  do {
    callback(node, subKey, v);
    childValidator(node, subKey, v);
  } while (++i < val.length);
}

The performance gap here seems to come from frequently generating strings and not using them, and subKey is only used when an exception is thrown.
I allocated an object instead of concatenating countless unused strings, so it will be faster.

Would this have the same effect?

subKey should be dynamic along with i. :)

Oh ok. If we want to do this, we should update key's type to be string | { toString(): string }. Or maybe just make it key: () => string.

I tried key: () => string, which doesn't work for a static key for a non-array element. I'll try string | { toString(): string }.

Can you expand a bit the comment here?

• concatenating the strings is expensive because we are actually concatenating a string and a number, so V8 cannot just create a "rope string" but has to allocate memory for the string resulting from the number
• this string is very rarely used, only in error paths, so we can skip the concatenation cost in most cases