express 5.2.1: Handle Express request header evaluation boundaries across req.is body presence, req.accepts zero-weight quality factors, req.range numeric sentinel codes, and req.get empty-string validation

# Express Request Header Evaluation Boundaries

## What `search_known_solution` Answered
`search_known_solution` returned existing published samples:
- `sha256:9ea5467ed4af202f10dc8512480582653e609d8f25f6ab7051747d6a4be46c37`: "Handle Express 5 silent behavior changes in default query parsing, numeric res.send/res.json status handling, and res.redirect('back')"
- `sha256:67b90276f29cdea20393f311586cc202c244b1f200d7a06896f5d072b0b912c1`: "Handle Express JSON body parser strictness and response serialization boundaries across primitives, strings, Buffers, and cookies"
- `sha256:d8b4f6e74d8b2973ca15dc5b87b356bcfb53694d16523889a26d0063ccfe53fd`: "Pass WHATWG URL objects or stringifiable non-string values to res.location and res.redirect without triggering TypeError"
- `sha256:91407f3e83bfceb9b5d36d67130853425cc7c6e45d5ac8532a6043cae432b0a7`: "Enable express trust proxy to resolve client IP, HTTPS protocol, and forwarded host behind reverse proxies without silently ignoring X-Forwarded headers"
- `sha256:f8ca69ed2b8293632a86541498c8a534e2e59536a647c9c4e6f3d7cd2ad45f88`: "Access parent route parameters inside nested express.Router instances using mergeParams: true"

## What a Model Would Have Written Instead
A model often assumes `req.is('json')` inspects the `Content-Type` header unconditionally and returns `'json'` or truthy on GET requests, assumes `req.range()` returns `null` or an empty array when a requested byte range is invalid or unsatisfiable, assumes `req.accepts()` accepts any media type listed in the `Accept` header even when weighted with `q=0`, and assumes `req.get('')` or `req.get(null)` safely yields `undefined`.

## How the Wrong Version Fails
It fails silently with incorrect branching when `req.is()` yields `null` on bodiless requests or when `req.range()` yields numeric error sentinels `-1` and `-2` (which evaluate to truthy in JavaScript conditionals `if (req.range(...))`), and fails loudly with a `TypeError: name argument is required to req.get` when passing empty or non-string header names to `req.get()`.

{"case":{"believed":"req.is('json') evaluates the Content-Type header directly regardless of whether a body was sent, req.range() returns empty arrays or null on invalid or unsatisfiable byte ranges, and req.accepts() returns true for any media type mentioned in the Accept header.","caseId":"case:sha256:1ca9893de226b5b04e0289e7ab41401fbb7c8a9fd64a9ae7d5c56ce3999f01bc","contract":["req.is('json') returns null when the request lacks body headers such as Content-Length or Transfer-Encoding even if Content-Type application/json is present, returning the matched type string only when a body is actually transmitted","req.accepts('json') returns false when the Accept header sets application/json;q=0, and multi-type negotiation req.accepts(['json', 'html']) skips zero-weight candidates to select acceptable alternatives","req.acceptsEncodings() returns false for all encodings when Accept-Encoding is set to *;q=0","req.range(size) returns numeric sentinel -1 for unsatisfiable byte ranges and numeric sentinel -2 for malformed range syntax rather than null or an empty array","req.range(size) returns undefined when the Range header is omitted, and parses valid suffix ranges such as bytes=-200 relative to the provided total size","req.get('') and req.get(null) throw TypeError requiring a non-empty string header name, while req.get('referrer') automatically aliases to the HTTP Referer header"],"goal":"Handle Express request header evaluation boundaries across req.is body presence, req.accepts zero-weight quality factors, req.range numeric sentinel codes, and req.get empty-string validation","kind":"HOW","packages":["pkg:npm/express@5.2.1"],"schemaVersion":1,"symbols":["req.is","req.accepts","req.acceptsEncodings","req.range","req.get"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","ecosystem":"npm","executionContext":"node","language":"node","os":"linux","packageManager":"npm","runtime":"node","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/express@5.2.1"],"schemaVersion":1,"symbols":["req.is","req.accepts","req.acceptsEncodings","req.range","req.get"],"verifierAdapter":"node-typescript@1"}

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}

{
  "name": "npm-express-d8",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "commonjs",
  "dependencies": {
    "express": "^5.2.1"
  }
}

import assert from 'node:assert';
import http from 'node:http';
import express from 'express';

const app = express();

app.all('/test/is', (req, res) => {
  res.json({
    hasHeader: req.get('content-type'),
    isJson: req.is('json'),
    isAppJson: req.is('application/json'),
    isText: req.is('text/plain'),
    isAny: req.is('*/*')
  });
});

app.get('/test/accepts', (req, res) => {
  res.json({
    acceptsNoArg: req.accepts(),
    acceptsJson: req.accepts('json'),
    acceptsHtml: req.accepts('html'),
    acceptsMulti: req.accepts(['json', 'html']),
    encodingsNoArg: req.acceptsEncodings(),
    encodingsGzip: req.acceptsEncodings('gzip'),
    encodingsIdentity: req.acceptsEncodings('identity')
  });
});

app.get('/test/range', (req, res) => {
  const size = parseInt(req.query.size || '1000', 10);
  const result = req.range(size);
  res.json({ result });
});

app.get('/test/get', (req, res) => {
  let emptyThrows = false;
  let nullThrows = false;
  try {
    req.get('');
  } catch (e) {
    if (e instanceof TypeError) emptyThrows = true;
  }
  try {
    req.get(null);
  } catch (e) {
    if (e instanceof TypeError) nullThrows = true;
  }

  res.json({
    emptyThrows,
    nullThrows,
    referer: req.get('referer'),
    referrer: req.get('referrer'),
    customCase: req.get('X-ExPrEsS-tEsT')
  });
});

const server = app.listen(0, async () => {
  const port = server.address().port;

  function request(options, body = null) {
    return new Promise((resolve, reject) => {
      const req = http.request({ port, host: '127.0.0.1', ...options }, (res) => {
        let data = '';
        res.on('data', chunk => {
          data += chunk;
        });
        res.on('end', () => {
          let json = null;
          try {
            json = JSON.parse(data);
          } catch {}
          resolve({ status: res.statusCode, headers: res.headers, text: data, json });
        });
      });
      req.on('error', reject);
      if (body !== null) {
        req.write(body);
      }
      req.end();
    });
  }

  try {
    // 1. req.is contract
    // Bodiless request returns null despite Content-Type header
    const getNoBody = await request({
      method: 'GET',
      path: '/test/is',
      headers: { 'content-type': 'application/json' }
    });
    assert.strictEqual(
      getNoBody.json.isJson,
      null,
      'req.is returns null when request has no body headers despite Content-Type being present'
    );
    assert.strictEqual(getNoBody.json.isAppJson, null);

    // Request with body returns matched type string
    const postWithBody = await request(
      {
        method: 'POST',
        path: '/test/is',
        headers: {
          'content-type': 'application/json; charset=utf-8',
          'content-length': '2'
        }
      },
      '{}'
    );
    assert.strictEqual(postWithBody.json.isJson, 'json');
    assert.strictEqual(postWithBody.json.isAppJson, 'application/json');
    assert.strictEqual(postWithBody.json.isText, false);
    assert.strictEqual(postWithBody.json.isAny, 'application/json');

    // 2. req.accepts contract
    // Absent Accept header returns ['*/*'] and matches any type
    const noAccept = await request({ path: '/test/accepts' });
    assert.deepStrictEqual(noAccept.json.acceptsNoArg, ['*/*']);
    assert.strictEqual(noAccept.json.acceptsJson, 'json');

    // Accept: application/json;q=0, text/html;q=0.5 (q=0 explicitly rejects json)
    const qZero = await request({
      path: '/test/accepts',
      headers: { accept: 'application/json;q=0, text/html;q=0.5' }
    });
    assert.strictEqual(qZero.json.acceptsJson, false, 'req.accepts returns false for types with q=0');
    assert.strictEqual(qZero.json.acceptsHtml, 'html');
    assert.strictEqual(
      qZero.json.acceptsMulti,
      'html',
      'req.accepts([json, html]) skips q=0 json and selects html'
    );

    // Accept-Encoding: *;q=0 rejects all encodings
    const qZeroEnc = await request({
      path: '/test/accepts',
      headers: { 'accept-encoding': '*;q=0' }
    });
    assert.strictEqual(qZeroEnc.json.encodingsGzip, false);
    assert.strictEqual(qZeroEnc.json.encodingsIdentity, false);

    // 3. req.range contract
    // Valid range returns array of ranges
    const validRange = await request({
      path: '/test/range?size=1000',
      headers: { range: 'bytes=0-499' }
    });
    assert.deepStrictEqual(validRange.json.result, [{ start: 0, end: 499 }]);

    // Suffix range bytes=-200
    const suffixRange = await request({
      path: '/test/range?size=1000',
      headers: { range: 'bytes=-200' }
    });
    assert.deepStrictEqual(suffixRange.json.result, [{ start: 800, end: 999 }]);

    // Unsatisfiable range returns numeric sentinel -1
    const unsatisfiable = await request({
      path: '/test/range?size=1000',
      headers: { range: 'bytes=2000-3000' }
    });
    assert.strictEqual(
      unsatisfiable.json.result,
      -1,
      'req.range returns -1 for unsatisfiable ranges'
    );

    // Malformed range returns numeric sentinel -2
    const malformed = await request({
      path: '/test/range?size=1000',
      headers: { range: 'bytes=foo-bar' }
    });
    assert.strictEqual(
      malformed.json.result,
      -2,
      'req.range returns -2 for malformed range syntax'
    );

    // Absent range returns undefined
    const absentRange = await request({ path: '/test/range?size=1000' });
    assert.strictEqual(absentRange.json.result, undefined);

    // 4. req.get contract
    const getHeaderRes = await request({
      path: '/test/get',
      headers: {
        referer: 'http://example.com/source',
        'X-eXpReSs-TeSt': 'valid-case'
      }
    });
    assert.strictEqual(getHeaderRes.json.emptyThrows, true, 'req.get("") throws TypeError');
    assert.strictEqual(getHeaderRes.json.nullThrows, true, 'req.get(null) throws TypeError');
    assert.strictEqual(getHeaderRes.json.referer, 'http://example.com/source');
    assert.strictEqual(
      getHeaderRes.json.referrer,
      'http://example.com/source',
      'req.get("referrer") aliases referer header'
    );
    assert.strictEqual(getHeaderRes.json.customCase, 'valid-case');
  } finally {
    server.close();
  }
});