How to Convert YAML to JSON in JavaScript (Node.js)

import { readFileSync } from "node:fs";
import YAML from "yaml";

const data = YAML.parse(readFileSync("app.yaml", "utf8"));
const json = JSON.stringify(data, null, 2);

With js-yaml it is load(text) instead of YAML.parse(text). js-yaml 5 has no default export, so import it as import { load } from "js-yaml". On the command line, the same package works as a one-liner:

node -e 'const YAML = require("yaml"); process.stdout.write(JSON.stringify(YAML.parse(require("fs").readFileSync(0, "utf8")), null, 2) + "\n")' < app.yaml

country: no
mode: 0755
released: 2026-10-18
version: 1.10
base: &base
  retries: 3
service:
  <<: *base
  name: checkout

{"country":"no","mode":755,"released":"2026-10-18","version":1.1,
 "base":{"retries":3},"service":{"<<":{"retries":3},"name":"checkout"}}

• country stays the string no, because both packages follow YAML 1.2 by default.
• mode becomes 755. YAML 1.2 writes octal as 0o755, so a leading zero is just a decimal number. If the value is a file permission, it is now wrong.
• released stays a string. YAML 1.2's core rules have no date type.
• version becomes the number 1.1. Quote version strings in YAML files.
• The merge key << is not applied. service contains a literal key named << instead of retries.

YAML.parse(text, { merge: true }).service
// { retries: 3, name: "checkout" }

YAML.parse(text, { version: "1.1" })
// country: false, mode: 493, released: "2026-10-18T00:00:00.000Z", merge keys applied

merge: true applies << merge keys, which many existing YAML files rely on, Docker Compose and CI configs among them. version: "1.1" reproduces how older tools read the file: no becomes false, 0755 is octal (493), and dates become Date objects, which JSON.stringify writes as ISO strings. Use it when the file was written for a YAML 1.1 tool and you need the same meaning. Python's PyYAML also follows YAML 1.1 rules.

A YAML file can hold several documents separated by ---, which Kubernetes manifests often do. YAML.parse refuses such a file with Source contains multiple documents; please use YAML.parseAllDocuments(). Convert each document and decide on the JSON shape, usually an array:

const docs = YAML.parseAllDocuments(text).map((doc) => doc.toJS());
// js-yaml: loadAll(text)
JSON.stringify(docs);   // [{"name":"a"},{"name":"b"}]

Both packages reject a mapping with the same key twice: yaml reports Map keys must be unique and js-yaml reports duplicated mapping key. That is stricter than JSON.parse, which keeps the last value without complaint. In a hand-edited config, the error is useful: it usually means a block was pasted twice.