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Minimal YAML to JSON Converter

A Node script that parses a small, common subset of YAML (key-value pairs, nested maps, and simple lists) and prints the equivalent JSON document.

node --version

node yaml2json.mjs sample.yaml

node --test yaml2json.test.mjs

Prints the JSON representation of the YAML file with nested maps and lists preserved.

What it does

Parses a deliberately small subset of YAML using indentation, which is enough for simple configuration files and shows how indentation-sensitive parsing works.

import { readFileSync } from 'node:fs';

function parseYaml(text) {
  const lines = text.split(/\r?\n/).filter((l) => l.trim() && !l.trim().startsWith('#'));
  const root = {};
  const stack = [{ indent: -1, value: root }];
  for (const line of lines) {
    const indent = line.match(/^ */)[0].length;
    const content = line.trim();
    while (stack.length > 1 && indent <= stack[stack.length - 1].indent) stack.pop();
    const parent = stack[stack.length - 1].value;
    if (content.startsWith('- ')) {
      if (!Array.isArray(parent)) throw new Error('list without a key: ' + content);
      parent.push(coerce(content.slice(2)));
      continue;
    }
    const [key, ...rest] = content.split(':');
    const value = rest.join(':').trim();
    if (value === '') {
      const child = {};
      parent[key.trim()] = child;
      stack.push({ indent, value: child });
    } else parent[key.trim()] = coerce(value);
  }
  return root;
}

function coerce(value) {
  if (value === 'true' || value === 'false') return value === 'true';
  if (/^-?\d+$/.test(value)) return Number(value);
  return value.replace(/^["']|["']$/g, '');
}

console.log(JSON.stringify(parseYaml(readFileSync(process.argv[2] ?? 'sample.yaml', 'utf8')), null, 2));

It handles scalars, nested maps, and dash lists, and explicitly does not attempt anchors, multi-line scalars, or flow syntax.

Run and verify

printf 'name: demo\nport: 3000\ntags:\n  - a\n  - b\n' > sample.yaml
node yaml2json.mjs sample.yaml

Create the small YAML sample then convert it; nesting is driven entirely by indentation.

Expected Output

For the sample the program should print JSON with the number, string, and list preserved. The output is:

{
  "name": "demo",
  "port": 3000,
  "tags": [
    "a",
    "b"
  ]
}

Numeric scalars become numbers, booleans become booleans, and nested keys are placed inside their parent map rather than flattened.

Limitations

It supports a small subset only; anchors, block scalars, flow collections, and quoted keys are out of scope and need a full parser.