yaml 2.9.0: Asynchronously traverse and modify YAML AST with visitAsync

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: Asynchronously traverse and modify YAML AST with visitAsync
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:npm/yaml@2.9.0
Demonstrate these symbols/APIs:
  - visitAsync

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:584503da0c90f459f86c9923c29d6e386e004018b44de84f0f0273026e62757a","contract":["assert visitAsync asynchronously traverses YAML AST nodes including Document, Map, Seq, Pair, and Scalar","assert visitAsync modifies Scalar node values and replaces nodes during async traversal","assert visitAsync removes nodes and key-value pairs when visitor returns visitAsync.REMOVE","assert visitAsync halts traversal immediately when visitor returns visitAsync.BREAK","assert visitAsync skips child node inspection when visitor returns visitAsync.SKIP"],"goal":"Asynchronously traverse and modify YAML AST with visitAsync","kind":"HOW","packages":["pkg:npm/yaml@2.9.0"],"schemaVersion":1,"symbols":["visitAsync"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"npm","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"npm","schemaVersion":1,"virtualization":"wsl"},"license":"MIT-0","packages":["pkg:npm/yaml@2.9.0"],"schemaVersion":1,"symbols":["visitAsync"],"verifierAdapter":"node-typescript@1"}

import { visitAsync, isScalar, isPair, isMap, isSeq, isDocument, Scalar } from 'yaml';

/**
 * Asynchronously updates all Scalar values matching the predicate.
 */
export async function transformScalarsAsync(doc, transformFn) {
  await visitAsync(doc, {
    async Scalar(key, node, path) {
      const updated = await transformFn(node.value, key, path);
      if (updated !== undefined) {
        node.value = updated;
      }
    }
  });
  return doc;
}

/**
 * Asynchronously removes pairs matching the filter condition.
 */
export async function filterPairsAsync(doc, shouldRemoveFn) {
  await visitAsync(doc, {
    async Pair(key, node, path) {
      const shouldRemove = await shouldRemoveFn(node.key?.value, node.value, path);
      if (shouldRemove) {
        return visitAsync.REMOVE;
      }
    }
  });
  return doc;
}

/**
 * Asynchronously collects visited node types.
 */
export async function collectNodeTypesAsync(doc) {
  const types = [];
  await visitAsync(doc, {
    async Map() { types.push('Map'); },
    async Seq() { types.push('Seq'); },
    async Pair() { types.push('Pair'); },
    async Scalar() { types.push('Scalar'); }
  });
  return types;
}

export { visitAsync, isScalar, isPair, isMap, isSeq, isDocument, Scalar };

{
  "name": "sample",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "sample",
      "version": "1.0.0",
      "dependencies": {
        "yaml": "2.9.0"
      }
    },
    "node_modules/yaml": {
      "version": "2.9.0",
      "resolved": "https://registry.npmjs.org/yaml/-/yaml-2.9.0.tgz",
      "integrity": "sha512-2AvhNX3mb8zd6Zy7INTtSpl1F15HW6Wnqj0srWlkKLcpYl/gMIMJiyuGq2KeI2YFxUPjdlB+3Lc10seMLtL4cA=="
    }
  }
}

{
  "name": "sample",
  "version": "1.0.0",
  "type": "module",
  "dependencies": {
    "yaml": "2.9.0"
  }
}

{
  "schemaVersion": 1,
  "goal": "Asynchronously traverse and modify YAML AST with visitAsync",
  "kind": "HOW",
  "packages": [
    "pkg:npm/yaml@2.9.0"
  ],
  "symbols": [
    "visitAsync"
  ]
}

import assert from 'node:assert/strict';
import { parseDocument, Scalar } from 'yaml';
import {
  visitAsync,
  transformScalarsAsync,
  filterPairsAsync,
  collectNodeTypesAsync
} from '../index.mjs';

// 1. assert visitAsync asynchronously traverses YAML AST nodes including Document, Map, Seq, Pair, and Scalar
{
  const yamlText = `
service: gateway
endpoints:
  - /health
  - /metrics
retries: 5
`;
  const doc = parseDocument(yamlText);
  const types = await collectNodeTypesAsync(doc);

  assert.ok(types.includes('Map'), 'Must visit Map nodes');
  assert.ok(types.includes('Seq'), 'Must visit Seq nodes');
  assert.ok(types.includes('Pair'), 'Must visit Pair nodes');
  assert.ok(types.includes('Scalar'), 'Must visit Scalar nodes');

  const visitedValues = [];
  await visitAsync(doc, {
    async Scalar(key, node) {
      visitedValues.push(node.value);
    }
  });

  assert.ok(visitedValues.includes('gateway'));
  assert.ok(visitedValues.includes('/health'));
  assert.ok(visitedValues.includes('/metrics'));
  assert.ok(visitedValues.includes(5));
}

// 2. assert visitAsync modifies Scalar node values and replaces nodes during async traversal
{
  const yamlText = `
user: alice
endpoint: /v1/api
`;
  const doc = parseDocument(yamlText);

  await transformScalarsAsync(doc, async (value) => {
    if (value === 'alice') {
      return Promise.resolve('bob');
    }
    return undefined;
  });

  // Replace a Scalar node with a new Scalar node
  await visitAsync(doc, {
    async Scalar(key, node) {
      if (node.value === '/v1/api') {
        return new Scalar('/v2/api');
      }
    }
  });

  const js = doc.toJS();
  assert.equal(js.user, 'bob');
  assert.equal(js.endpoint, '/v2/api');
}

// 3. assert visitAsync removes nodes and key-value pairs when visitor returns visitAsync.REMOVE
{
  const yamlText = `
environment: production
debugMode: true
tags:
  - legacy
  - stable
`;
  const doc = parseDocument(yamlText);

  await filterPairsAsync(doc, async (key) => {
    return Promise.resolve(key === 'debugMode');
  });

  await visitAsync(doc, {
    async Scalar(key, node) {
      if (node.value === 'legacy') {
        return visitAsync.REMOVE;
      }
    }
  });

  const js = doc.toJS();
  assert.equal(js.debugMode, undefined);
  assert.equal(js.environment, 'production');
  assert.deepEqual(js.tags, ['stable']);
}

// 4. assert visitAsync halts traversal immediately when visitor returns visitAsync.BREAK
{
  const yamlText = `
first: 10
second: 20
third: 30
fourth: 40
`;
  const doc = parseDocument(yamlText);
  const seenScalars = [];

  await visitAsync(doc, {
    async Scalar(key, node) {
      seenScalars.push(node.value);
      if (node.value === 20) {
        return visitAsync.BREAK;
      }
    }
  });

  assert.ok(seenScalars.includes('first'));
  assert.ok(seenScalars.includes(10));
  assert.ok(seenScalars.includes('second'));
  assert.ok(seenScalars.includes(20));
  assert.ok(!seenScalars.includes('third'), 'Should break before third');
  assert.ok(!seenScalars.includes(30), 'Should break before 30');
  assert.ok(!seenScalars.includes('fourth'), 'Should break before fourth');
  assert.ok(!seenScalars.includes(40), 'Should break before 40');
}

// 5. assert visitAsync skips child node inspection when visitor returns visitAsync.SKIP
{
  const yamlText = `
metadata:
  nestedField1: skipMe
  nestedField2: skipMeToo
keepField: processMe
`;
  const doc = parseDocument(yamlText);
  const processed = [];

  await visitAsync(doc, {
    async Map(key, node, path) {
      if (path.length > 0 && path[path.length - 1].key?.value === 'metadata') {
        return visitAsync.SKIP;
      }
    },
    async Scalar(key, node) {
      processed.push(node.value);
    }
  });

  assert.ok(processed.includes('metadata'));
  assert.ok(!processed.includes('nestedField1'), 'nestedField1 must be skipped');
  assert.ok(!processed.includes('skipMe'), 'skipMe must be skipped');
  assert.ok(!processed.includes('nestedField2'), 'nestedField2 must be skipped');
  assert.ok(!processed.includes('skipMeToo'), 'skipMeToo must be skipped');
  assert.ok(processed.includes('keepField'));
  assert.ok(processed.includes('processMe'));
}

console.log('PASS: All visitAsync contract assertions passed successfully');