Create scroll-based animation without JavaScript.

Sometimes we want to update the CSS style of an HTML element based on the scroll position, just like fast-forwarding or rewinding a video by scrolling up and down.

With Trigger JS, get the desired value with CSS variable on page scroll for your animation needed, without writing a single line of JavaScript code, configuration with HTML attributes. Checkout examples here.

Read this document in other languages: English, 繁體中文, 简体中文.

1. Include Trigger JS to your webpage with a script tag, with either CDN:

   • UNPKG CDN:

   <script src="//unpkg.com/@triggerjs/trigger" defer></script>

   • jsDelivr CDN:

   <script src="//cdn.jsdelivr.net/npm/@triggerjs/trigger" defer></script>

2. Add tg-name to the DOM element that you want to monitor. The value of tg-name is the name of the CSS variable that binds to the element.

<div tg-name="scrolled" id="greeting">Hello, World</div>

In the above example, CSS variable --scrolled is added to the selector #greeting:

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough room for scrolling */
  }

  #greeting {
    transform: translateX(
      calc(var(--scrolled) * 1px)
    ); /* Converts to px unit */
  }
</style>

3. Scroll the page and see the result.

1. Get the library in either way:
   • From GitHub

   git clone https://github.com/triggerjs/trigger.git

   • From NPM

   npm i @triggerjs/trigger

2. Change to the directory, install the dependencies:

   npm install

3. There is a pre-built version bundle.js located in dist. Run a local web server and browse the greeting example in index.html :
   1. For example, type npx serve in the terminal
   2. Open up http://localhost:5000 in web browser.
   3. Scroll the page and see the result.
4. The following command will build a new version to dist/bundle.js:
   • For development (with watch):

     npm run watch

   • For development:

     npm run build

   • For production:

     npm run prod

Number is not suitable for all the situations. For example, we want to update the text color based on the scroll value. the attribute tg-map can help.

The following example shows how to update the text color with the rules below:

<h1
  id="heading"
  tg-name="color"
  tg-from="1"
  tg-to="10"
  tg-steps="9"
  tg-map="1: black; 2: red; 3: orange; 4: yellow; 5: green; 6: cyan; 7: blue; 8: purple; 9,10: grey"
>
  Rainbow Text
</h1>

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough rooms for scrolling */
  }

  #heading {
    color: var(--color);
  }
</style>

Let's say tg-from="200" and tg-to="-200", we want to move the element in x position with transform: translateX(). tg-steps lets us define how many steps from 200 to -200, for example, tg-steps="400" means run from 200 to -200 with 400 steps, 1 per increment; In other words, tg-steps="800" means 0.5 per increment.

But sometimes, we do not want to do the math by ourselves, that's why tg-step exists. tg-step defines the exact value of increment. Please note that if tg-step is defined, tg-steps will be ignored.

Sometimes we are only interested in certain values. For example, we only want to know when 25, 50, 75 show up from 0 to 100 (tg-from="0" and tg-to="100"). In this situation, tg-filter helps you.

<h1
  id="heading"
  tg-name="color"
  tg-from="0"
  tg-to="100"
  tg-step="1"
  tg-filter="25,50,75"
  tg-map="25: red; 50: yellow; 75: green"
>
  Red (25), Yellow (50), Green (75)
</h1>

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough rooms for scrolling */
  }

  #heading {
    color: var(--color);
  }
</style>

There are two modes for tg-filter, retain by default, the other one is exact. Here is an example to clarify this:

<h1
  id="heading"
  tg-name="color"
  tg-from="0"
  tg-to="10"
  tg-step="1"
  tg-filter="5"
  tg-map="5: blue"
>
  Trigger.js
</h1>

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough rooms for scrolling */
  }

  #heading {
    --color: black;
    color: var(--color);
  }
</style>

In the above example, the text has an initial color of black, and it will turn to blue when it arrives at the middle of the page and never turn to black again because there is no trigger point of the black color.

So let's say we want the text color becomes blue only when the calculation value is 5, and becomes black for other values, We can change it to:

<h1
  id="heading"
  tg-name="color"
  tg-from="0"
  tg-to="10"
  tg-step="1"
  tg-filter="4,5,6"
  tg-map="4: black; 5: blue; 6: black"
>
  Trigger.js
</h1>

It works, but the code becomes redundant. To solve this, we can switch the filter mode to exact by adding an ! at the end of the value of tg-filter:

<h1
  id="heading"
  tg-name="color"
  tg-from="0"
  tg-to="10"
  tg-step="1"
  tg-filter="5!"
  tg-map="5: blue"
>
  Trigger.js
</h1>

In exact mode, --color becomes blue when the value is 5, and becomes the default when the value is not 5.

The design of adding ! to the value of tg-filter is the demand is exclusive to the attribute. Establishing another attribute for the mode is unnecessary or even leads to the misunderstanding.

Just like some CSS properties, the values of tg- attributes (except tg-follow, tg-ref) inherits from the parents if not being set in the current element. If we do not want it inherits from parent and set it as default value, just add the tg- attribute without value. For example:

<div tg-name="scale" tg-from="0" tg-to="50">
  <span tg-name="color" tg-to>
    <!-- The value of tg-to is now 1 (Default value) -->
  </span>
</div>

The different between cover (default) and edge:

So that if tg-edge="inset", the element must be higher than the viewport (window.clientHeight).

We can also listen to the tg event on an element with JavaScript:

<h1
  id="heading"
  tg-name="color"
  tg-from="1"
  tg-to="3"
  tg-steps="2"
  tg-map="1:#000;2:#666;3:#ccc"
>
  Trigger JS
</h1>

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough room for scrolling */
  }

  #heading {
    color: var(--color);
  }
</style>

<script>
  document.querySelector('#heading').addEventListener('tg', (e) => {
    console.log(e.detail); // {value: '#666'}
  });
</script>

If you are concerned with the tg- prefix that doesn't quite fulfill the standard of HTML5, it can be customised by the following setting in the body tag with data-trigger-prefix attribute:

<body data-trigger-prefix="data-tg">
  <div data-tg-name="scrolled" id="greeting">Hello, World</div>
</body>

The above example customises the prefix to data-tg. data-* is a completely valid attribute for putting custom data in HTML5.

Feel free to fork this repository and submit pull requests. Bugs report in GitHub Issues, features/ideas/questions discuss in GitHub Discussions.

Trigger.js is MIT Licensed.