YAML vs JSON: A Comparison

Introduction

YAML vs JSON is a common comparison for developers, DevOps engineers and data architects when it comes to choosing a format for configuration, data exchange or system integration. Both are lightweight, human-readable serialisation languages that structure data using key-value pairs, lists and nested elements. While JSON is known for its strict syntax and popularity in APIs and web development, YAML is favoured in configuration files and DevOps workflows due to its clean, indentation-based format. This blog post explains the key differences, benefits and best use cases for each format.

Why data serialisation formats are important

In the modern development landscape, applications rarely exist in isolation. They communicate with each other, read configurations and exchange data across systems. Data serialisation formats play a decisive role in this. They help to structure and store data in such a way that it can be read by both humans and machines.

The triumph of YAML and JSON

Among the various formats, YAML and JSON have emerged as two of the most popular. Developers often encounter them in configuration files, API responses, infrastructure as code and data pipelines. Their simplicity and readability make them the ideal choice in many use cases.

Purpose of this comparison

Although both formats serve a similar purpose, they have different advantages and disadvantages. This article will highlight the differences between YAML and JSON to help you understand when you should use which format depending on your project requirements.

What is JSON?

Overview of JSON

JSON (JavaScript Object Notation) is a lightweight format for data exchange. Originally derived from JavaScript, it is now a language-independent format that is supported by most programming languages. JSON is known for its simplicity and is therefore often used for APIs, configuration files and data storage.

Basic syntax of JSON

JSON represents data as key-value pairs in objects ({}) and ordered lists in arrays ([]). All keys must be strings enclosed in quotes, and the values can be strings, numbers, Boolean values, nulls, arrays or other objects.

{
 "name": "Alice",
 "age": 30,
 "isMember": true,
 "hobbies": ["reading", "cycling"]
}

Where JSON is commonly used

JSON is the de facto standard for data exchange on the web. It is used in REST APIs, configuration files for web applications and cloud service integrations. Its compatibility with JavaScript and broad support for parsers make it ideal for frontend-backend communication and data transfer.

What is YAML?

YAML at a glance

YAML stands for “YAML Ain’t Markup Language”.” It is a human-friendly standard for serialising data that is designed for readability and user-friendliness. YAML is often used to write configuration files and data that should be easily understood and edited by humans. The syntax is largely based on indentation, which makes it visually clear and concise.

Basic syntax of YAML

YAML uses indentation to represent structures, making brackets and commas superfluous. Enumerations are represented with hyphens, key-value pairs with colons. Inverted commas are optional unless they are required for special characters.

name: Alice
age: 30 isMember: true
hobbies:
 - reading
 - cycling

Where YAML is commonly used

YAML is very popular in DevOps and cloud-native environments. It is commonly used in configuration files for tools such as Kubernetes, Docker Compose, Ansible, GitHub Actions and more. YAML’s readability and simple syntax make it the first choice for teams managing infrastructure, pipelines and declarative settings.

Syntax comparison

Visual structure

JSON and YAML differ significantly in their visual structure. JSON uses curly braces ({}) and square brackets ([]) to define objects and arrays and requires double quotes around keys and string values. YAML, on the other hand, uses indentation to represent hierarchies, which results in less visual noise but increases the susceptibility to formatting errors.

JSON example:

{
 "server": {
 "host": "localhost",
 "port": 8080
 },
 "enabled": true
}

YAML example:

server:
    host: localhost
    port: 8080 enabled: true

Delimiters and punctuation

JSON uses commas, colons and brackets throughout, making the structure strict and easier to analyse by machine. YAML dispenses with most punctuation, which improves readability but increases the susceptibility to errors due to missing indentations or incorrectly arranged spaces.

Important JSON requirements:

Keys must be enclosed in inverted commas
Commas are used between elements
Curly brackets are required

Important YAML features:

No quotes required unless necessary
No commas between elements
Relies solely on indentation

Multiline and complex structures

The handling of multi-line strings or nested structures differs between the two. YAML offers more syntactic possibilities, such as block literals (|) and folded style (>), which make it more expressive for human-readable data. JSON has no built-in syntax for multi-line strings and requires escaping of characters.

YAML multiline example:

description: |
 This is a multiline
 block of text that
  retains line breaks.

JSON equivalent:

{
 "description": "This is a multiline\nblock of text that\nretains line breaks."
}

Support of data types

Common data types in JSON

JSON supports a limited but widely used number of data types:

String: Enclosed in double quotes
Numbers: Integers and floating point values
Booleans: true or false
Null: Represents empty or missing values
Arrays: Ordered list of values
Objects: Unordered collection of key-value pairs

Example:

{
 "name": "Alice",
 "age": 30,
 "active": true,
 "score": null,
 "skills": ["JavaScript", "Python"]
}

Common data types in YAML

YAML supports all core data types found in JSON and also allows more complex types and custom tagging. The YAML type system includes:

Strings: In single, single or double inverted commas
Numbers: Integers, floating point numbers, octal numbers, hexadecimal numbers
Booleans: true, false, yes, no
Null: Represented as null, ~ or empty
Lists: Labelled with hyphens
Dictionaries: Key-value pairs, separated by colons

Example:

name: Alice
age: 30 
active: true 
score: ~
skills:
 - JavaScript
 - Python

Flexibility and customisation of type

YAML offers more flexibility in type definition and interpretation. It even supports user-defined types through tags (e.g. !!str, !!int), which can be used to explicitly declare the data type of a value. JSON does not allow user-defined types or annotations, which makes it more limited but safer and more predictable for parsing.

YAML:

version: !!float 1.0 
count: !!int "42"

Readability and user-friendliness

Readability for people

YAML is designed to be easy to read for humans. The clean and minimal syntax, indentation-based structure and lack of unnecessary punctuation make it feel more like a natural language. Whilst JSON is structured and consistent, it can be difficult to read in deeply nested or large files due to the brackets and inverted commas.

Example comparison:

JSON:

{
 "user": {
 "name": "Alice",
 "roles": ["admin", "editor"],
 "active": true
 }
}

YAML:

user:
 name: Alice
 roles:
 - admin
 - editor
 active: true

Error proneness

The dependence on indentation makes YAML error-prone, especially for beginners. A missing space or incorrect indentation can lead to unexpected behaviour or parsing errors. JSON is stricter but more predictable — errors are usually due to missing commas or untruncated characters, which are easily recognised by parsers.

Learning curve

JSON has an easier learning curve, especially for developers familiar with JavaScript or similar languages. While YAML is more readable, it has additional features such as anchors, aliases and different string styles that can be confusing at first. The flexibility of YAML can sometimes prove to be a disadvantage when it comes to consistency.

Editing experience

Editing JSON is easier in environments with syntax validation, autocomplete and formatters. YAML requires more care when editing manually due to indentation, but is better suited for writing short, structured documents without the need for a specialised tool.

Tooling and ecosystem

Language support and libraries

JSON is natively supported by almost every modern programming language. Languages such as JavaScript, Python, Java, Go and Ruby have integrated functions or libraries for parsing and generating JSON data. YAML also has broad language support, but often requires third-party libraries such as PyYAML (Python), ruamel.yaml, js-yaml (JavaScript) or SnakeYAML (Java).

JSON:

const obj = JSON.parse('{"name": "Alice"}');

YAML:

import yaml

with open("config.yaml") as file:
 config = yaml.safe_load(file)

Integration of editor and IDE

Most modern IDEs and text editors provide excellent support for JSON and YAML, including syntax highlighting, formatting, linting and schema validation. JSON benefits from more mature tooling due to its longer presence in web development. YAML is also well supported, especially in DevOps-focused environments such as VS Code, IntelliJ and JetBrains IDEs.

Schema and validation tools

JSON Schema is a mature and widely used standard for defining and validating the structure of JSON data. YAML can use the same JSON schema for validation, especially in tools like Kubernetes, but its flexible syntax can make validation difficult.

JSON: Strong support for automated validation and API documentation
YAML: Compatible with JSON schema, but more difficult to validate due to optional typing and formatting flexibility

Popular tools and frameworks

JSON is often used in web technologies, APIs and databases such as MongoDB. YAML is favoured in infrastructure and configuration tools.

JSON-based tools:

RESTful APIs
Postman
Firebase
ElasticSearch

YAML-based tools:

Kubernetes
Docker Compose
GitHub Actions
Ansible

The ecosystem for both formats is rich, but their strengths are focussed on different types of workflows: JSON for data transport, YAML for configuration.

Use cases and industry adoption

JSON in web development and APIs

JSON is the standard for transferring data between web clients and servers. It is the standard format used in RESTful APIs and JavaScript-based applications due to its tight integration with front-end frameworks. JSON is also commonly used in mobile applications, cloud platforms and serverless architectures.

Common use cases include:

REST API responses and requests
Configuration for JavaScript libraries (e.g., package.json)
NoSQL databases such as MongoDB
Local browser storage and cookies

YAML in configuration and DevOps

YAML is the preferred format for writing configuration files in DevOps and Infrastructure-as-Code environments. Its human-friendly syntax makes it ideal for scenarios where developers and system administrators need to read and edit configurations frequently.

Common use cases are:

Kubernetes manifests (deployment.yaml)
Docker Compose configurations
CI/CD workflows in GitHub Actions and GitLab
Infrastructure definitions in Ansible and SaltStack

Industry preferences and deployment trends

Tech startups and web app developers often lean towards JSON due to integration with web frameworks and APIs.
DevOps engineers, SREs and platform teams tend to favour YAML for its expressiveness and clean syntax in configuration-heavy tools.
Cloud providers such as AWS, Azure and Google Cloud support both formats, although YAML is often more prevalent in templates and declarative setups.

While both formats co-exist in modern tech stacks, the choice often depends on the problem domain —JSON is favoured for data exchange, while YAML is preferred for human-readable configuration.

Safety considerations

Secure parsing and execution

JSON is generally safer to parse because it has a stricter syntax and does not support references, aliases or custom types. Most JSON parsers treat the data as plain text and do not execute embedded code, making them suitable for untrusted input to web applications.

YAML, on the other hand, is more expressive, but also more risky. Some YAML parsers support functions such as references, anchors and user-defined object deserialisation, which can lead to security vulnerabilities if handled carelessly.

Common vulnerabilities in YAML

YAML’s advanced features can lead to security vulnerabilities when parsing untrusted input. These include:

Any code execution: Some YAML parsers (e.g. yaml.load in Python) can deserialise into arbitrary objects, which attackers can exploit.
Denial of Service (DoS): Deeply nested YAML structures can exhaust system resources, leading to service outages.
Information leak: Aliases and anchors can inadvertently expose sensitive internal structures if misused.

Secure YAML parsing typically involves the use of restricted parsers (e.g. safe_load) and the avoidance of features such as custom tags or object deserialisation.

Best practises for secure use

To minimise security risks, developers should follow these practises:

Use safe and restricted parsers (safe_load, json.loads, etc.)
Avoid executing or evaluating content parsed from YAML or JSON
Limit file sizes and nesting levels to prevent DoS attacks
Validate the structure with a schema if possible
Sanitise or filter input if external YAML or JSON files are accepted

When it comes to security, especially in environments that process user input, JSON is usually the more secure standard due to its limited scope and low risk of code execution. YAML should be used with special care and strict parser settings.

Final Thoughts: Which ones should you use?

Selection by use case

The best format depends on what you want to achieve. JSON is ideal for data transfer, APIs and storage where structure and strict typing are important. YAML lends itself to environments where readability and ease of editing are paramount, such as configuration files and infrastructure definitions.

Use JSON if:

You work with web APIs or front-end applications
Speed and consistency of machine processing are crucial
Data needs to be exchanged frequently between services

Use YAML if:

You need a readable configuration format
Files are managed and edited by humans
You work with DevOps tools or infrastructure-as-code

Compromises and developer preferences

JSON offers simplicity, consistency and better tools for validation and integration. It is easier to debug, especially for large systems, due to its predictable syntax. YAML is more expressive and cleaner to write, but can be more difficult to debug and validate due to indentation sensitivity and flexible formatting.

Some teams prefer JSON because of its rigidity and compatibility with linting and schema validation tools. Others favour YAML for its elegance in managing nested configurations and avoiding visual clutter.

When should you mix both formats?

In some projects you may use both formats. JSON is often used in API responses, while YAML is used for configuration files. Tools like Kubernetes accept JSON equivalents of YAML files, and many systems can convert between the two formats using CLI tools or libraries.

Knowing the strengths and weaknesses of both methods will help you make a smart choice depending on your specific workflows and team requirements.