Raw HTTP Header Block Parser

const HEADER_NAME = /^[!#$%&'*+\-.^_\x60|~0-9a-z]+$/;

export class HeaderBlock {
  #fields = new Map();

  add(name, value) {
    const key = name.toLowerCase();
    if (!HEADER_NAME.test(key)) throw new SyntaxError("invalid header name: " + name);
    const current = this.#fields.get(key);
    if (current === undefined) {
      this.#fields.set(key, value);
    } else if (key === "set-cookie") {
      const list = Array.isArray(current) ? current : [current];
      this.#fields.set(key, [...list, value]);
    } else {
      this.#fields.set(key, current + ", " + value);
    }
  }

  append(name, value) {
    const key = name.toLowerCase();
    const current = this.#fields.get(key);
    if (current === undefined) throw new SyntaxError("continuation line without a header");
    if (Array.isArray(current)) {
      const last = current[current.length - 1];
      this.#fields.set(key, [...current.slice(0, -1), last + " " + value]);
    } else {
      this.#fields.set(key, current + " " + value);
    }
  }

  get(name) {
    const value = this.#fields.get(name.toLowerCase());
    if (value === undefined) return null;
    return Array.isArray(value) ? value.join(", ") : value;
  }

  getAll(name) {
    const value = this.#fields.get(name.toLowerCase());
    if (value === undefined) return [];
    return Array.isArray(value) ? [...value] : [value];
  }

  has(name) {
    return this.#fields.has(name.toLowerCase());
  }

  names() {
    return [...this.#fields.keys()];
  }

  toObject() {
    const output = Object.create(null);
    for (const [name, value] of this.#fields) {
      output[name] = Array.isArray(value) ? [...value] : value;
    }
    return output;
  }
}

export function parseHeaderBlock(raw, options = {}) {
  if (typeof raw !== "string") throw new TypeError("raw headers must be a string");
  const { maxLines = 200, maxBytes = 64 * 1024 } = options;
  if (!Number.isInteger(maxLines) || maxLines < 1) {
    throw new RangeError("maxLines must be a positive integer");
  }
  if (!Number.isInteger(maxBytes) || maxBytes < 1) {
    throw new RangeError("maxBytes must be a positive integer");
  }
  const byteLength = new TextEncoder().encode(raw).length;
  if (byteLength > maxBytes) throw new RangeError("header block exceeds " + maxBytes + " bytes");

  const lines = raw.replace(/\r\n?/g, "\n").split("\n");
  const headers = new HeaderBlock();
  let lastName = null;
  let lineCount = 0;
  for (const line of lines) {
    if (line === "") break;
    lineCount += 1;
    if (lineCount > maxLines) throw new RangeError("header block exceeds " + maxLines + " lines");
    if (/^[ \t]/.test(line)) {
      if (lastName === null) throw new SyntaxError("continuation line without a header");
      headers.append(lastName, line.trim());
      continue;
    }
    const colon = line.indexOf(":");
    if (colon <= 0) throw new SyntaxError("invalid header line: " + line);
    const name = line.slice(0, colon).trim();
    const value = line.slice(colon + 1).trim();
    headers.add(name, value);
    lastName = name.toLowerCase();
  }
  return headers;
}