Implement Uint8Array base64/hex encoding methods (fromBase64, fromHex, toBase64, toHex, setFromBase64, setFromHex)

[Nakshatra480]

Description

Boa is missing the Uint8Array base64 and hex encoding/decoding methods specified in ECMAScript 2025 (Section 23.3). These methods provide native, efficient conversion between Uint8Array byte data and base64/hex string representations — functionality that is extremely common in real-world JavaScript (handling binary data, cryptographic digests, network protocols, etc.).

Methods to implement

Method	Spec Section	Type
Uint8Array.fromBase64(string [, options])	§23.3.1.1	Static
Uint8Array.fromHex(string)	§23.3.1.2	Static
Uint8Array.prototype.toBase64([options])	§23.3.2.3	Instance
Uint8Array.prototype.toHex()	§23.3.2.4	Instance
Uint8Array.prototype.setFromBase64(string [, options])	§23.3.2.1	Instance
Uint8Array.prototype.setFromHex(string)	§23.3.2.2	Instance

Helper abstract operations (§23.3.3)

• ValidateUint8Array(ta) — §23.3.3.1
• GetUint8ArrayBytes(ta) — §23.3.3.2
• SetUint8ArrayBytes(into, bytes) — §23.3.3.3
• SkipAsciiWhitespace(string, index) — §23.3.3.4
• DecodeFinalBase64Chunk(chunk, throwOnExtraBits) — §23.3.3.5
• DecodeFullLengthBase64Chunk(chunk) — §23.3.3.6
• FromBase64(string, alphabet, lastChunkHandling [, maxLength]) — §23.3.3.7
• FromHex(string [, maxLength]) — §23.3.3.8

Expected Behavior

// Encoding
const bytes = new Uint8Array([72, 101, 108, 108, 111]);
bytes.toBase64();  // "SGVsbG8="
bytes.toHex();     // "48656c6c6f"

// Decoding
Uint8Array.fromBase64("SGVsbG8=");        // Uint8Array [72, 101, 108, 108, 111]
Uint8Array.fromHex("48656c6c6f");         // Uint8Array [72, 101, 108, 108, 111]

// Options: base64url alphabet
bytes.toBase64({ alphabet: "base64url" });
Uint8Array.fromBase64("SGVsbG8", { alphabet: "base64url", lastChunkHandling: "loose" });

// In-place set operations
const target = new Uint8Array(10);
target.setFromBase64("SGVsbG8=");   // returns { read: 8, written: 5 }
target.setFromHex("48656c6c6f");    // returns { read: 10, written: 5 }

Implementation Hints

• Location: core/engine/src/builtins/typed_array/ — either extend the existing Uint8Array builtin or add a new uint8array_extras.rs module
• Rust ecosystem: The base64 and hex crates can accelerate implementation, but the spec algorithms should be followed precisely for edge cases (whitespace handling, lastChunkHandling option, alphabet selection)
• Test262 coverage: The test262 suite includes tests under test/built-ins/Uint8Array/fromBase64/, test/built-ins/Uint8Array/fromHex/, test/built-ins/Uint8Array/prototype/toBase64/, test/built-ins/Uint8Array/prototype/toHex/, etc.
• Other engines: V8, SpiderMonkey, and JavaScriptCore all ship these methods

Why this matters

Base64/hex encoding is ubiquitous in JavaScript — from fetch response handling to WebCrypto to working with binary protocols. Until these methods existed, JS developers had to rely on atob/btoa (which work on strings, not bytes) or third-party libraries. These spec-mandated methods are the correct, type-safe, standards-compliant way to do it.

[jedel1043]

Should be solved by #4630