Parse options and schemas in yaml

The same YAML text can mean different things depending on the schema and YAML version, and parse options let you choose which rules apply.

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Parse options and schemas in yaml: screenshot of the example below running — The code below, running in Chromium on our server (Oct 12, 2026). Press Run to see it live and change it.

How it works

Integers as BigInt. With { intAsBigInt: true } the number 999 comes back as a bigint instead of a number. [1]
Failsafe schema. { schema: 'failsafe' } keeps scalars as strings, so 999 is returned as the string '999'. [1]
YAML 1.1 booleans and the json schema. By default No is a string. With version: '1.1' or schema: 'yaml-1.1' it becomes false, and the json schema rejects it, so the code catches the error. [1]

Run and edit it

It runs in your browser, in a sandboxed frame that cannot see this site; the packages load from esm.sh only after you press Run.

import { parse } from 'yaml'

const show = (label, value) =>
  console.log(label.padEnd(36), String(value), '(' + typeof value + ')')

// integers
show("parse('number: 999')", parse('number: 999').number)
show('intAsBigInt: true', parse('number: 999', { intAsBigInt: true }).number)
show("schema: 'failsafe'", parse('number: 999', { schema: 'failsafe' }).number)

// YAML 1.2 core schema vs YAML 1.1
show("parse('No')", parse('No'))
show("version: '1.1'", parse('No', { version: '1.1' }))
show("schema: 'yaml-1.1'", parse('No', { schema: 'yaml-1.1' }))
show("parse('3')", parse('3'))
show("parse('3', failsafe)", parse('3', { schema: 'failsafe' }))

// the json schema only accepts JSON-style scalars
try {
  parse('No', { schema: 'json' })
} catch (err) {
  console.log("schema: 'json' error:", err.message.split('\n')[0])
}

Ctrl/⌘ + Enter runs

Try other values such as 'yes', 'on' or '0o14' with and without version: '1.1'. Code written by AgentGid [1].

Checked Oct 12, 2026: it starts without errors in Chromium with yaml 2.9.1. Needs WebGL.

More yaml examples

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Sources

yaml website

Explanations written by AgentGid from the official docs; the numbers in brackets point here. The code marked "written by AgentGid" is ours and free to copy.

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