What Is Mermaid.js?
Mermaid.js is an open-source diagramming tool that allows users to create diagrams and charts using plain text. Instead of drawing diagrams manually with graphical editors, Mermaid.js lets you describe diagrams using a simple, readable syntax and renders them automatically as visual diagrams.
This approach is often referred to as diagrams as code, and it has become increasingly popular in software documentation, system design, and technical communication.
A Brief History of Mermaid.js
Mermaid.js was created to solve a common problem faced by developers and technical writers: keeping diagrams up to date. Traditional diagram tools often produce binary files or images that are difficult to version, review, and maintain.
By defining diagrams in text form, Mermaid.js makes diagrams easy to store in version control systems like Git, review in pull requests, and update alongside code and documentation.
Today, Mermaid.js is widely used for software architecture diagrams, flowcharts, sequence diagrams, and technical documentation.
How Mermaid.js Works
At a high level, Mermaid.js works in three steps:
- Parse text-based diagram definitions — you write a diagram using Mermaid syntax in plain text.
- Convert syntax into a diagram structure — Mermaid.js parses the text and builds an internal representation.
- Render the diagram visually — the diagram is rendered as SVG, making it sharp and scalable for the web.
Because Mermaid diagrams are rendered from text, the same diagram definition can be reused across different tools, platforms, and documents.
Mermaid.js Syntax Explained (Simple Example)
Mermaid.js syntax is designed to be simple and readable. Instead of focusing on drawing shapes manually, you focus on structure and logic. Mermaid.js handles layout and rendering automatically.
This simplicity makes Mermaid.js especially useful for technical documentation, README files, architecture discussions, and system and workflow diagrams.
Common Diagram Types in Mermaid.js
Mermaid.js supports a wide range of diagram types, including:
- Flowcharts
- Sequence diagrams
- Class diagrams
- State diagrams
- Entity relationship diagrams
- Gantt charts
Each diagram type uses its own syntax, but all follow the same principle: define structure in text, then render visually.
Why Mermaid.js Is Popular in Documentation
Mermaid.js has become a standard tool in documentation workflows for several reasons:
- Version control friendly — diagrams are stored as text
- Easy to update — no need to redraw diagrams manually
- Consistent output — diagrams stay visually consistent
- Tool-agnostic — works across many platforms and editors
The Importance of Previewing Mermaid Diagrams
While Mermaid.js defines the structure of a diagram, visual appearance still matters. The same Mermaid diagram can look very different depending on theme, spacing, layout style, line rendering, and background color.
Without previewing, diagrams may look fine in one context but appear crowded or unclear in another — especially in presentations or architecture reviews.
Mermaid Themes and Layouts
Mermaid.js supports themes that control colors, fonts, and overall styling. Layout options influence spacing, density, and how diagrams flow visually. Choosing the right combination helps ensure diagrams are readable, visually balanced, and appropriate for their audience.
For example: hand-drawn styles work well for early design discussions, neutral themes are better for documentation and reports, and presentation-focused layouts improve clarity on slides.
How Mermaid Viewer Fits into the Workflow
Mermaid Viewer is designed to complement Mermaid.js by focusing on preview, styling, and layout. If you already write Mermaid syntax, Mermaid Viewer helps you see the final result clearly before sharing or exporting diagrams.
When to Use Mermaid.js
Mermaid.js is a good choice when you want to keep diagrams close to your code or documentation, collaborate using version control, maintain diagrams over time, and focus on structure rather than manual drawing.
Summary
Mermaid.js is a text-based diagramming language that makes diagrams easier to write, maintain, and share. Previewing Mermaid diagrams is a key part of the workflow, and tools like Mermaid Viewer help ensure diagrams look clear, readable, and ready for their intended use.