YAML to JSON Converter

What each YAML feature becomes in JSON

JSON has fewer types than YAML, so some things change on the way. Every result below comes from this converter:

• Comments are dropped. JSON has no place for them.
• | keeps line breaks: a script: | block with npm ci and npm test gives "npm ci\nnpm test\n". > folds the lines into one, with spaces between them. Adding - (|- or >-) removes the final line break.
• ~, null, Null, NULL and an empty value all become null. Only true and false (also True, TRUE and so on) become booleans.
• Numbers: 0x1F gives 31, 0o755 gives 493, and 6.02e23 gives 6.02e+23. Trailing zeros go, so 1.10 becomes 1.1; quote version numbers. YAML 1.2 doesn't allow _ in numbers, so 1_000_000 stays the string "1_000_000".
• Integers of any size stay exact: user_id: 12345678901234567890 comes out as 12345678901234567890, not a rounded value.
• .inf and .nan become null, because JSON has no infinity or NaN.
• Explicit tags work: !!str 123 gives the string "123".
• Anchors (&name) and aliases (*name) are written out as full copies, because JSON has no references. A tiny file with nested aliases can expand to gigabytes (the "billion laughs" attack), so the converter stops above 200,000 values.

Why YAML parsers disagree

YAML 1.1 (2005) reads many plain words as booleans, numbers or dates. The YAML 1.2 core schema (2009) keeps only true, false and null (plus a few capitalizations and ~) as special words. Many tools still use 1.1 rules, for example PyYAML and sigs.k8s.io/yaml, the Go library behind Kubernetes, so one file can produce different data in different tools.

In YAML 1.2, octal needs the 0o prefix: 0o755 gives 493, while 0755 is the decimal 755. If a value must stay text, such as a country code, a version like 1.10, a postal code with a leading zero or a port mapping, quote it. The JSON to YAML converter adds those quotes for you.

Common YAML errors

Each message below is what the converter shows, followed by the line and column:

• A tab used for indentation: tab characters must not be used in indentation. YAML allows only spaces.
• One space too many or too few: bad indentation of a mapping entry. Keys at the same level need exactly the same indentation.
• A value with a colon and a space, like title: Note: read me: bad indentation of a mapping entry (line 1, column 12). Put the value in quotes.
• A value that starts with *: cron: */15 * * * * gives unidentified alias "/15" (line 1, column 11), because * starts an alias. Quote it.
• An inline list that isn't closed, like key: [1, 2 followed by another key: missed comma between flow collection entries.
• The same key twice in one mapping: duplicated mapping key, pointing at the second one.
• A custom tag such as CloudFormation's !Ref: BucketName: !Ref MyBucket gives unknown tag !<!Ref> (line 1, column 26). Write the long form, Ref: MyBucket, instead.
• A missing space after the colon gives no error at all, which is worse: name:web becomes the single string "name:web", not a key and a value.

Convert YAML to JSON in code

Watch for dates and multiple documents. PyYAML turns 2024-01-15 into a date that json.dumps can't write without default=str, and safe_load fails on a file with more than one document, so use safe_load_all there. js-yaml's default schema turns the same date into "2024-01-15T00:00:00.000Z"; pass CORE_SCHEMA to keep it as text. Ruby's safe_load refuses dates unless you allow the Date class.

import json, yaml

with open("config.yaml") as f:
    data = yaml.safe_load(f)
print(json.dumps(data, indent=2, default=str))

const fs = require("fs");
const yaml = require("js-yaml");

const data = yaml.load(fs.readFileSync("config.yaml", "utf8"), {
  schema: yaml.CORE_SCHEMA,
});
console.log(JSON.stringify(data, null, 2));

yq -o=json '.' config.yaml

import "sigs.k8s.io/yaml"

j, err := yaml.YAMLToJSON(yamlBytes)

require "date"
require "json"
require "yaml"

data = YAML.safe_load(File.read("config.yaml"), permitted_classes: [Date], aliases: true)
puts JSON.pretty_generate(data)