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pnpm add tinyaml

tinyaml ships ESM only. Node 22.12 and later load it with import or require(), and so do bundlers.

import { parse, parseAll, stringify } from "tinyaml";

parse("title: Hello\ntags: [a, b]\ncount: 0x1F"); // { title: "Hello", tags: ["a", "b"], count: 31 }
parseAll("a: 1\n---\nb: 2"); // [{ a: 1 }, { b: 2 }]
stringify({ title: "Hello", draft: false, answer: "yes" }); // "title: Hello\ndraft: false\nanswer: 'yes'\n"

Values follow the YAML 1.2 core schema. true, null, ~, .5, 0o17, 0x1F and .inf become values, and yes, on and dates stay strings.

Merge keys work and are on by default:

parse("base: &base { a: 1 }\nchild:\n  <<: *base\n  b: 2");
// { base: { a: 1 }, child: { a: 1, b: 2 } }

Custom tags are functions that get the node's value, and they can call parse themselves. A tag you don't handle keeps its value instead of throwing:

parse("home: !env HOME", {
  tags: { "!env": (value) => (typeof value === "string" ? process.env[value] : undefined) },
});
parse("x: !thing 12"); // { x: "12" }

Errors are YAMLErrors with a 1-based line and column, also at the end of the message:

parse("a: 1\na: 2"); // throws YAMLError: duplicated mapping key (2:1)

stringify writes what JSON.stringify would keep, with a few differences. Functions, symbols and undefined are left out of objects. In arrays they become null, and so do holes. toJSON is called with no key, so a Date becomes an ISO string, and a toJSON that returns undefined leaves the value out. A BigInt throws. Numbers JSON can't write become YAML ones: Infinity is .inf, NaN is .nan and -0 is -0.0. Any other object is written from its own enumerable keys, so a Map or a boxed new Number(5) becomes {}. It quotes strings a YAML 1.1 reader would take for something else, so "yes" comes out as 'yes' and "0777" as '0777'.

Collection keys such as [a, b]: c throw on purpose. A list can't be a JavaScript object key without being turned into a string.