YAML Formatter

How it works

JSON ↔ YAML at a Glance

Every JSON file is valid YAML. The two formats represent the same data — just with different syntax.

{
  "name": "web-app",
  "replicas": 3,
  "enabled": true,
  "tags": ["web", "prod"],
  "limits": {
    "cpu": "500m",
    "memory": "256Mi"
  }
}

name: web-app
replicas: 3
enabled: true
tags:
  - web
  - prod
limits:
  cpu: 500m
  memory: 256Mi

{
  "apiVersion": "apps/v1",
  "kind": "Deployment",
  "metadata": {
    "name": "web-app"
  },
  "spec": {
    "replicas": 3,
    "template": {
      "spec": {
        "containers": [{
          "name": "web",
          "image": "nginx:1.25",
          "ports": [{"containerPort": 80}]
        }]
      }
    }
  }
}

apiVersion: apps/v1
kind: Deployment
metadata:
  name: web-app
spec:
  replicas: 3
  template:
    spec:
      containers:
        - name: web
          image: nginx:1.25
          ports:
            - containerPort: 80

YAML Quick Reference

message: |
  Line one.
  Line two.
  Line three.

description: >
  This long sentence wraps
  across several lines but
  becomes one paragraph.

defaults: &defaults
  timeout: 30
  retries: 3

production:
  <<: *defaults
  host: prod.example.com

staging:
  <<: *defaults
  host: staging.example.com

defaults:
  timeout: 30
  retries: 3

production:
  timeout: 30
  retries: 3
  host: prod.example.com

staging:
  timeout: 30
  retries: 3
  host: staging.example.com

YAML 1.1 vs YAML 1.2 — The Hidden Trap

Why the same YAML file gives different results in Python vs JavaScript

services:
  worker:
    environment:
      FEATURE_FLAG: yes        # ⚠ Docker reads this as boolean true, not string "yes"
      DEPLOY_TIME: 2024-06-01  # ⚠ PyYAML converts this to a datetime object
      PORT: 22:30              # ⚠ go-yaml v2 reads this as integer 1350

Tips & Tricks

Using YAML in Code

How to parse, format, and convert YAML programmatically in the most common environments.

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

// Parse a YAML file into a JavaScript object
const config = yaml.load(fs.readFileSync('config.yaml', 'utf8'));

// Serialize a JavaScript object to a YAML string
const yamlStr = yaml.dump(config, { indent: 2, lineWidth: -1 });

// Convert a JSON file to YAML
const json = JSON.parse(fs.readFileSync('config.json', 'utf8'));
fs.writeFileSync('config.yaml', yaml.dump(json, { indent: 2 }));

import yaml, json

# Parse a YAML file into a Python dict
with open('config.yaml') as f:
    config = yaml.safe_load(f)

# Serialize a Python dict to YAML (block style)
yaml_str = yaml.dump(config, default_flow_style=False, indent=2, allow_unicode=True)

# Convert a JSON file to YAML
with open('config.json') as f:
    data = json.load(f)
with open('config.yaml', 'w') as f:
    yaml.dump(data, f, default_flow_style=False, indent=2)

from ruamel.yaml import YAML

ryaml = YAML()
ryaml.preserve_quotes = True

# Parse — comments and formatting are preserved
with open('config.yaml') as f:
    config = ryaml.load(f)

# Write back — comments are kept intact
with open('config.yaml', 'w') as f:
    ryaml.dump(config, f)

# Pretty-print / validate a YAML file
yq '.' config.yaml

# Convert YAML → JSON
yq -oj config.yaml

# Convert JSON → YAML
yq -oy config.json

# Read a single value
yq '.metadata.name' deployment.yaml

# In-place update
yq -i '.spec.replicas = 5' deployment.yaml

# Merge two YAML files
yq '. * load("override.yaml")' base.yaml

# Lint with default rules
yamllint config.yaml

# Relaxed ruleset (fewer style warnings)
yamllint -d relaxed config.yaml

# Lint all YAML files in the current directory
yamllint -d relaxed .

# Use in GitHub Actions
- name: Lint YAML
  run: pip install yamllint && yamllint -d relaxed .

Frequently Asked Questions

What does YAML stand for?

YAML stands for YAML Ain't Markup Language — a recursive acronym. It was originally called "Yet Another Markup Language" when it was created in 2001, but was later renamed to distinguish it from document markup languages like HTML and XML. YAML is a data serialization format, not a markup language.

What is YAML used for?

YAML is used for configuration files and data that humans need to read and write. Common use cases include: CI/CD pipeline definitions (GitHub Actions, GitLab CI, CircleCI), container orchestration (Docker Compose, Kubernetes), infrastructure-as-code (Ansible, Helm charts), application configuration (Rails, Spring Boot), and API specifications (OpenAPI/Swagger). Its clean indentation-based syntax makes it easier to read than JSON or XML for deeply nested structures.

YAML vs JSON — what is the difference and which should I use?

YAML and JSON both represent structured data, but serve different purposes. YAML supports comments, multi-line strings, anchors, and is much easier to read and hand-edit — making it the standard for config files. JSON has no comments, stricter syntax, and is universally supported by APIs and programming languages — making it ideal for data exchange. JSON is actually a valid subset of YAML 1.2, so any JSON file is also valid YAML. Use YAML for files humans write and maintain; use JSON for data sent between systems.

What is a YAML formatter?

A YAML formatter takes raw or compact YAML text and re-indents it consistently so the structure is easy to read. It also validates the YAML syntax and reports any errors with the exact line and column number.

Is my YAML data sent to a server?

No. All formatting, validation, and conversion happens entirely in your browser using JavaScript. Your data never leaves your machine.

Can I convert YAML to JSON?

Yes. Click the → JSON button to convert your YAML input to formatted JSON in the output panel. You can also click ← JSON to convert JSON input to YAML.

Why is my YAML invalid?

Common YAML errors include: mixing tabs and spaces for indentation, missing space after the colon in key: value, unclosed flow collections like [1, 2 or {a: 1, and unquoted strings that look like other types. When your input is invalid, the status bar shows the exact line and column number of the error and highlights that line in the gutter.

Does YAML support tab indentation?

No. The YAML specification explicitly prohibits tab characters for indentation — they are only allowed inside quoted strings. This formatter only offers 2-space and 4-space options, which covers all valid YAML documents.

What is the difference between YAML 1.1 and YAML 1.2?

YAML 1.1 (used by PyYAML, Docker Compose, GitHub Actions) treats bare yes, no, on, off, y, and n as booleans, interprets 22:30 as the integer 1350 (sexagesimal), and auto-converts date strings like 2024-01-15 into Date objects. YAML 1.2 (the current standard) only recognises true and false as booleans. This mismatch causes silent data corruption when YAML written for one parser is consumed by another.

What are YAML anchors and aliases?

Anchors (&name) mark a node so it can be reused, and aliases (*name) reference that node later. This avoids duplicating identical blocks across your config. The Tools → Expand Anchors button resolves all anchors and <<: merge keys into their final values, useful when your target parser does not support anchors.

How do I write multi-line strings in YAML?

Use the literal block scalar (|) to preserve every newline exactly, or the folded block scalar (>) to fold newlines into spaces. Append - (e.g. |-) to strip the trailing newline, which is useful for shell scripts embedded in CI config.

How do I keep a value like 1.0 or 0755 as a string?

Wrap it in quotes. Unquoted 1.0 is parsed as the float 1.0 (and serialised as 1 by most libraries). Unquoted 0755 is treated as octal (493) in YAML 1.1. Always write version: '1.0' and mode: '0755' to guarantee string type across all parsers.

Can I use this formatter for GitHub Actions or Docker Compose files?

Yes. Paste your workflow or Compose file, click Beautify to format it, and then use Tools → Fix YAML 1.1 Risks to automatically quote any values that would be silently misinterpreted by those parsers. Both GitHub Actions and Docker Compose use YAML 1.1 parsers internally.

Related Tools

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