chore: refactor to consolidate base64 encoding functions (alternative)

[Rich-Harris]

#14157, but with a simpler implementation of base64_encode and base64_decode. I got rid of the omitPadding and alphabet options, I think they're unnecessary

---

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: f826d99

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[ottomated]

This works, but I think we still need the Buffer check for big files (which need to be considered because base64 is used for read):

(charCode array is this implementation, which is a few ns slower for small inputs but much faster for large inputs)

function base64_encode(bytes) {
  const binary = new Array(bytes.length);

  for (let i = 0; i < bytes.length; i++) {
    binary[i] = String.fromCharCode(bytes[i]);
  }

  return btoa(binary.join(""));
}

[Rich-Harris]

What tool are you using for benchmarking? Surprised the difference is so stark — locally it's more like 10x, which was still a surprise (and reason enough to use Buffer) but nothing like that screenshot.

In basically all cases that we can't use Buffer (i.e. the browser), we're dealing with small payloads. So I think it's better to avoid allocating that array

[ottomated]

That was mitata with a ~4MB file – I think the difference grows exponentially with the file size