Trigger JS

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.

Getting Started

Method 1: Via CDN

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.

Method 2: Build from source

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

The tg- Attributes

Attribute	Type	Default	Description
tg-name	Required	-	The CSS variable name to store the value, with or without -- prefix.
tg-from	Optional	0	The start value
tg-to	Optional	1	The end value
tg-steps	Optional	100	Steps to be triggered from tg-from to tg-to
tg-step	Optional	0	Step per increment. If this value isn't 0, will override tg-steps.
tg-map	Optional	(Empty)	Map the value to another value. Format: - 1-to-1 mapping: value: newValue; value2: newValue2. - Multiple-to-1 mapping: value,value2,value3: newValue. - Range-to-1 mapping: value...value2: newValue.
tg-filter	Optional	(Empty)	Only trigger if the scroll value is on the list. Format: 1,3,5,7,9. By default, the filter mode is retain. If we want to switch the mode to exact, add an ! at the end of the value. Read more about this in the dedicated section following.
tg-edge	Optional	cover	Calculate the start and end of the scrolling effect. cover means off-screen to off-screen. The calculation starts in the appearance of the element at the bottom, and ends in the disappearance of element at the top; inset represents the calculation begins after the top edge of the element touches the top of the screen, ends when the bottom edge of the element reached the bottom of the screen. See below section for a diagram.
tg-follow	Optional	(Empty)	Use the result calculated from another element. The value of tg-follow is the value of the target element's tg-ref. Caution: When tg-follow is set, tg-from, tg-to, tg-steps, tg-step and tg-edge are ignored in the same element.
tg-ref	Optional	(Empty)	Define the name for other elements to reference using tg-follow.
tg-bezier	Optional	(Empty)	Bezier easing setting, available values: ease, easeIn, easeOut, easeInOut, or custom numbers for a Cubic Bezier in format p1x,p1y,p2x,p2y.

Value Mapping

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:

Element Position (From the Bottom)	Scroll Value	Text Color
0% - 10%	1	black
10% - 20%	2	red
20% - 30%	3	orange
30% - 40%	4	yellow
40% - 50%	5	green
50% - 60%	6	cyan
60% - 70%	7	blue
70% - 80%	8	purple
80% - 90%	9	grey
90% - 100%	10	grey

<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>

Steps & Step

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.

Noise Reduction

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>

The mode of tg-filter

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.

Value Inheritance

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>

tg-edge Explaination

The different between cover (default) and edge:

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

JavaScript Event

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>

Customising the Prefix

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.

Contribute

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

License

Trigger.js is MIT Licensed.