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Usage

Mermaid is a JavaScript tool that makes use of a Markdown based syntax to render customizable diagrams, charts and visualizations.

Diagrams can be re-rendered/modified by modifying their descriptions.

CDN

https://www.jsdelivr.com/package/npm/mermaid

Please note that you can switch versions through the dropdown box at the top right.

Using mermaid

For the majority of users, Using the Live Editor would be sufficient, however you may also opt to deploy mermaid as a dependency or using the Mermaid API.

We have compiled some Video Tutorials on how to use the mermaid Live Editor.

Installing and Hosting Mermaid on a Webpage

Using the npm package

1. You will need to install node v16, which would have npm.

2. download yarn using npm.

3. enter the following command:
    yarn add mermaid

4. At this point, you can add mermaid as a dev dependency using this command:
    yarn add --dev mermaid

5. Alternatively, you can also deploy mermaid using the script tag in an HTML file with mermaid diagram descriptions.
    as is shown in the example below

Hosting mermaid on a web page.

Note:This topic explored in greater depth in the User Guide for Beginners

The easiest way to integrate mermaid on a web page requires two elements:

  • A graph definition, inside <pre> tags labeled class=mermaid. Example:
html
<pre class="mermaid">
    graph LR
    A --- B
    B-->C[fa:fa-ban forbidden]
    B-->D(fa:fa-spinner);
</pre>
  • Inclusion of the mermaid address in the html page body using a script tag as an ESM import, and the mermaidAPI call.

Example:

html
<script type="module">
  import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@9/dist/mermaid.esm.min.mjs';
  mermaid.initialize({ startOnLoad: true });
</script>

Following these directions, mermaid starts at page load and (when the page has loaded) it will locate the graph definitions inside the pre tags with class="mermaid" and return diagrams in SVG form, following given definitions.

Simple full example:

html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <pre class="mermaid">
  graph LR
      A --- B
      B-->C[fa:fa-ban forbidden]
      B-->D(fa:fa-spinner);
    </pre>
    <script type="module">
      import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@9/dist/mermaid.esm.min.mjs';
      mermaid.initialize({ startOnLoad: true });
    </script>
  </body>
</html>

Notes:

An id attribute is also added to mermaid tags without one.

Mermaid can load multiple diagrams, in the same page.

Try it out, save this code as HTML and load it using any browser.(Except Internet Explorer, please don't use Internet Explorer.)

Enabling Click Event and Tags in Nodes

A securityLevel configuration has to first be cleared, securityLevel sets the level of trust for the parsed diagrams and limits click functionality. This was introduce in version 8.2 as a security improvement, aimed at preventing malicious use.

It is the site owner's responsibility to discriminate between trustworthy and untrustworthy user-bases and we encourage the use of discretion.

securityLevel

ParameterDescriptionTypeRequiredValues
securityLevelLevel of trust for parsed diagramStringRequired'sandbox', 'strict', 'loose', 'antiscript'

Values:

  • strict: (default) tags in text are encoded, click functionality is disabled
  • loose: tags in text are allowed, click functionality is enabled
  • antiscript: html tags in text are allowed, (only script element is removed), click functionality is enabled
  • sandbox: With this security level all rendering takes place in a sandboxed iframe. This prevent any JavaScript running in the context. This may hinder interactive functionality of the diagram like scripts, popups in sequence diagram or links to other tabs/targets etc.

Note This changes the default behaviour of mermaid so that after upgrade to 8.2, unless the securityLevel is not changed, tags in flowcharts are encoded as tags and clicking is disabled. sandbox security level is still in the beta version.

If you are taking responsibility for the diagram source security you can set the securityLevel to a value of your choosing . This allows clicks and tags are allowed.

To change securityLevel, you have to call mermaidAPI.initialize:

javascript
mermaidAPI.initialize({
  securityLevel: 'loose',
});

Labels out of bounds

If you use dynamically loaded fonts that are loaded through CSS, such as Google fonts, mermaid should wait for the whole page to load (dom + assets, particularly the fonts file).

javascript
$(document).load(function () {
  mermaid.initialize();
});

or

javascript
$(document).ready(function () {
  mermaid.initialize();
});

Not doing so will most likely result in mermaid rendering graphs that have labels out of bounds. The default integration in mermaid uses the window.load event to start rendering.

If your page has other fonts in its body those might be used instead of the mermaid font. Specifying the font in your styling is a workaround for this.

css
div.mermaid {
  font-family: 'trebuchet ms', verdana, arial;
}

Calling mermaid.init

By default, mermaid.init will be called when the document is ready, finding all elements with class="mermaid". If you are adding content after mermaid is loaded, or otherwise need finer-grained control of this behavior, you can call init yourself with:

  • a configuration object
  • some nodes, as
    • a node
    • an array-like of nodes
    • or W3C selector that will find your nodes

Example:

javascript
mermaid.init({ noteMargin: 10 }, '.someOtherClass');

Or with no config object, and a jQuery selection:

javascript
mermaid.init(undefined, $('#someId .yetAnotherClass'));

Warning This type of integration is deprecated. Instead the preferred way of handling more complex integration is to use the mermaidAPI instead.

Usage with webpack

mermaid fully supports webpack. Here is a working demo.

API usage

The main idea of the API is to be able to call a render function with the graph definition as a string. The render function will render the graph and call a callback with the resulting SVG code. With this approach it is up to the site creator to fetch the graph definition from the site (perhaps from a textarea), render it and place the graph somewhere in the site.

The example below show an outline of how this could be used. The example just logs the resulting SVG to the JavaScript console.

html
<script type="module">
  import mermaid from './mermaid.mjs';
  mermaid.mermaidAPI.initialize({ startOnLoad: false });
  $(async function () {
    // Example of using the API var
    element = document.querySelector('#graphDiv');
    const insertSvg = function (svgCode, bindFunctions) {
      element.innerHTML = svgCode;
    };
    const graphDefinition = 'graph TB\na-->b';
    const graph = await mermaid.mermaidAPI.render('graphDiv', graphDefinition, insertSvg);
  });
</script>

Binding events

Sometimes the generated graph also has defined interactions like tooltip and click events. When using the API one must add those events after the graph has been inserted into the DOM.

The example code below is an extract of what mermaid does when using the API. The example shows how it is possible to bind events to an SVG when using the API for rendering.

javascript
const insertSvg = function (svgCode, bindFunctions) {
  element.innerHTML = svgCode;
  if (typeof callback !== 'undefined') {
    callback(id);
  }
  bindFunctions(element);
};

const id = 'theGraph';

mermaidAPI.render(id, txt, insertSvg, element);
  1. The graph is generated using the render call.
  2. After generation the render function calls the provided callback function, in this case it's called insertSvg.
  3. The callback function is called with two parameters, the SVG code of the generated graph and a function. This function binds events to the SVG after it is inserted into the DOM.
  4. Insert the SVG code into the DOM for presentation.
  5. Call the binding function that binds the events.

Example of a marked renderer

This is the renderer used for transforming the documentation from Markdown to html with mermaid diagrams in the html.

javascript
const renderer = new marked.Renderer();
renderer.code = function (code, language) {
  if (code.match(/^sequenceDiagram/) || code.match(/^graph/)) {
    return '<pre class="mermaid">' + code + '</pre>';
  } else {
    return '<pre><code>' + code + '</code></pre>';
  }
};

Another example in CoffeeScript that also includes the mermaid script tag in the generated markup.

coffee
marked = require 'marked'

module.exports = (options) ->
  hasMermaid = false
  renderer = new marked.Renderer()
  renderer.defaultCode = renderer.code
  renderer.code = (code, language) ->
    if language is 'mermaid'
      html = ''
      if not hasMermaid
        hasMermaid = true
        html += '<script src="'+options.mermaidPath+'"></script>'
      html + '<pre class="mermaid">'+code+'</pre>'
    else
      @defaultCode(code, language)

  renderer

Advanced usage

Syntax validation without rendering (Work in Progress)

The mermaid.parse(txt) function validates graph definitions without rendering a graph. This function is still a work in progress, find alternatives below.

The function mermaid.parse(txt), takes a text string as an argument and returns true if the definition follows mermaid's syntax and false if it does not. The parseError function will be called when the parse function returns false.

When the parser encounters invalid syntax the mermaid.parseError function is called. It is possible to override this function in order to handle the error in an application-specific way.

The code-example below in meta code illustrates how this could work:

javascript
mermaid.parseError = function (err, hash) {
  displayErrorInGui(err);
};

const textFieldUpdated = function () {
  const textStr = getTextFromFormField('code');

  if (mermaid.parse(textStr)) {
    reRender(textStr);
  }
};

bindEventHandler('change', 'code', textFieldUpdated);

Alternative to mermaid.parse(): One effective and more future-proof method of validating your graph definitions, is to paste and render them via the Mermaid Live Editor. This will ensure that your code is compliant with the syntax of Mermaid's most recent version.

Configuration

Mermaid takes a number of options which lets you tweak the rendering of the diagrams. Currently there are three ways of setting the options in mermaid.

  1. Instantiation of the configuration using the initialize call
  2. Using the global mermaid object - Deprecated
  3. using the global mermaid_config object - Deprecated
  4. Instantiation of the configuration using the mermaid.init call- Deprecated

The list above has two ways too many of doing this. Three are deprecated and will eventually be removed. The list of configuration objects are described in the mermaidAPI documentation.

Using the mermaidAPI.initialize/mermaid.initialize call

The future proof way of setting the configuration is by using the initialization call to mermaid or mermaidAPI depending on what kind of integration you use.

html
<script src="../dist/mermaid.js"></script>
<script>
  let config = { startOnLoad: true, flowchart: { useMaxWidth: false, htmlLabels: true } };
  mermaid.initialize(config);
</script>

Note This is the preferred way of configuring mermaid.

The following methods are deprecated and are kept only for backwards compatibility.

Using the mermaid object

It is possible to set some configuration via the mermaid object. The two parameters that are supported using this approach are:

  • mermaid.startOnLoad
  • mermaid.htmlLabels
javascript
mermaid.startOnLoad = true;

Warning This way of setting the configuration is deprecated. Instead the preferred way is to use the initialize method. This functionality is only kept for backwards compatibility.

Using the mermaid_config

It is possible to set some configuration via the mermaid object. The two parameters that are supported using this approach are:

  • mermaid_config.startOnLoad
  • mermaid_config.htmlLabels
javascript
mermaid_config.startOnLoad = true;

Warning This way of setting the configuration is deprecated. Instead the preferred way is to use the initialize method. This functionality is only kept for backwards compatibility.

Using the mermaid.init call

To set some configuration via the mermaid object. The two parameters that are supported using this approach are:

  • mermaid_config.startOnLoad
  • mermaid_config.htmlLabels
javascript
mermaid_config.startOnLoad = true;

Warning This way of setting the configuration is deprecated. Instead the preferred way is to use the initialize method. This functionality is only kept for backwards compatibility.