YAML to JSON Converter and Validator

How to use it

1. Paste YAML, or choose “JSON to YAML” and paste JSON.
2. Choose the YAML version of the program that reads or wrote the file: 1.2 for current parsers, 1.1 for PyYAML and other older ones.
3. Read the result. An error names the line and column and selects the spot in the field.
4. Read the warnings: they list every value that the other YAML version would read differently, and what was changed to fit JSON.
5. Copy or download the result.

Worked example: one file, two readings

The same four lines give different data depending on the YAML version:

country: NO
mode: 0755
duration: 1:30
version: 3.10

YAML 1.2: {"country": "NO", "mode": 755, "duration": "1:30", "version": 3.1}
YAML 1.1: {"country": false, "mode": 493, "duration": 90, "version": 3.1}

In YAML 1.1 the word NO is a boolean (the “Norway problem”), 0755 is an octal number (493) and 1:30 is a base-60 number (90). YAML 1.2 reads the first and third as strings and 0755 as the decimal number 755; octal is written 0o755 there. Both versions read 3.10 as the number 3.1, so a version string needs quotes. The tool lists the first three as warnings, whichever version you choose.

How the conversion works

The tool uses the open-source yaml library (eemeli/yaml), which implements both the YAML 1.2 core schema and the YAML 1.1 types. It is loaded the first time you convert. The text is parsed twice, once for each version, and every unquoted value is compared; where the two readings differ, the tool shows both.

YAML can express things that JSON cannot. Keys that are not strings, such as 1 or true, become strings. Anchors and aliases (&name and *name) are expanded into copies, with a limit of 100 aliases against “billion laughs” files. Several documents separated by --- become a JSON array. Dates of YAML 1.1 become ISO 8601 strings, .inf and .nan become null, and integers keep every digit, even beyond the 2^53 limit of JavaScript numbers.

In the other direction the JSON is checked strictly first. The YAML output uses block style, keeps the numbers as written and puts quotes around every string that a YAML 1.1 or 1.2 reader would otherwise take for something else, such as "no", "0755" or "2001-12-14", so the file reads back as the same data in both versions. A key named << is quoted too, because YAML 1.1 would read it as a merge key.

YAML 1.1 and YAML 1.2 at a glance

The YAML 1.1 type definitions list y and n as booleans, but PyYAML, a widely used YAML 1.1 parser, reads them as strings. This tool’s 1.1 mode follows the type definitions, so its results for y and n differ from PyYAML. Quote such values and nothing depends on the parser.

The same on the command line

# yq (mikefarah/yq, v4)
yq -o=json config.yaml
yq -p=json -o=yaml config.json

# Python with PyYAML (YAML 1.1 rules)
python3 -c "import sys, json, yaml; print(json.dumps(yaml.safe_load(sys.stdin), indent=2, default=str))" < config.yaml

Common mistakes

• Leaving country codes, answers or switches unquoted: NO, yes and off turn into booleans for YAML 1.1 parsers.
• Writing file modes or ZIP codes with a leading zero, such as 0755 or 01234, without quotes.
• Writing version numbers such as 3.10 or 1.20 without quotes; they become 3.1 and 1.2.
• Indenting with tabs. YAML allows only spaces for indentation.
• Repeating a key in the same mapping. Some parsers keep the last value silently; this tool reports it as an error.

Sources

• YAML 1.2.2 specification, section 10.3: core schema
• YAML 1.1: boolean type
• YAML 1.1: integer type (octal and base 60)
• YAML 1.1: merge key
• yaml library for JavaScript (eemeli/yaml)
• RFC 8259: JSON