JSON and YAML: the same data model, different syntax

YAML 1.2 is a superset of JSON: every JSON document is also valid YAML, and both describe the same kinds of values (mappings, sequences, strings, numbers, booleans, null). Converting between them is therefore lossless for ordinary data. The differences are about syntax and typing rules, and that is where surprises come from.

JSON

{
  "name": "api",
  "ports": [8080, 9090],
  "debug": false,
  "owner": null
}

YAML

name: api
ports:
  - 8080
  - 9090
debug: false
owner: null

What to watch for when converting YAML to JSON

Implicit typing. In YAML, version: 1.10 is the number 1.1, zip: 01234 is a number in some parsers, and on, yes, no were booleans in YAML 1.1. This converter uses the YAML 1.2 core schema (via js-yaml's default), so yes and no stay strings. Quote values you want to keep as strings: version: "1.10".
Dates. An unquoted 2026-02-01 is parsed as a timestamp and serialised to JSON as an ISO 8601 string ("2026-02-01T00:00:00.000Z"). Quote it in the YAML if you want the literal text.
Anchors and aliases. &default / *default are resolved: the aliased content is copied into each place it is used, because JSON has no references.
Multiple documents. A file with --- separators becomes a JSON array with one element per document.
Comments are dropped. JSON cannot carry them.
Key order is preserved. Duplicate keys are an error in YAML, and this converter reports them.

What to watch for when converting JSON to YAML

Strings that look like other types are quoted automatically ("true", "123", "2026-02-01", "null"), so round-tripping keeps them as strings.
Multi-line strings use a literal block (|) which is easier to read than escaped \n sequences.
Long lines are not folded. Some YAML emitters wrap at 80 columns; that is legal but makes diffs noisy, so it is disabled here.
Empty objects and arrays become {} and [].

Converting in code

Python

import json, yaml   # pip install pyyaml
data = yaml.safe_load(open("config.yaml"))
print(json.dumps(data, indent=2))
# JSON -> YAML
print(yaml.safe_dump(json.load(open("data.json")), sort_keys=False))

Node.js

import yaml from "js-yaml";   // npm install js-yaml
const data = yaml.load(await fs.readFile("config.yaml", "utf8"));
console.log(JSON.stringify(data, null, 2));
console.log(yaml.dump(JSON.parse(text), { noRefs: true }));

Command line

yq -o=json . config.yaml       # mikefarah/yq
yq -P . data.json               # JSON -> YAML (pretty)

Notes on this implementation

Conversion runs in your browser using the js-yaml library (MIT licence), loaded only on this page. JSON parsing uses the same diagnosing parser as the JSON Validator, so syntax errors come with line, column and a hint. YAML errors report the line and column from the YAML parser.

JSON ↔ YAML Converter