kohaiy / trigger

A library for creating scroll-based animation with HTML attributes and CSS variables.

Trigger JS

Get the desired value with CSS variable on page scroll for your animation need, without writing a single line of JavaScript code.

Sometimes we want to create an animation or update the CSS style of an HTML element based on the scroll position, just like playing a video forward and backward by scrolling up and down.

Initially, it was just a number from 0 to 1 when the element appears from the bottom and disappears from the top. However, it is way too simple for various of animation needs, hence the range can be configured on a per-element basis. But numbers don't always work, with mapping and filtering support, we can map a number to a color or something else, for us to use it in CSS directly. Checkout some 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 script tag:

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

2. Add tg-name to the DOM element that you want to monitor. The value of tg-name will be the name of the CSS variable that bind to this element.

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

In the above example, CSS variable --scrolled will be available to #greeting:

<style>
  body {
    padding: 100vh 0; /* In order to make the page have enough rooms 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 already a pre-built version located in dist/bundle.js. Run a local web server and checkout index.html for the greeting example:
   1. For example, with npx serve
   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	How many steps between tg-from and tg-to
tg-step	Optional	0	Step per increment, if this value is other than 0, will override tg-steps.
tg-map	Optional	(Empty)	map the value to another value. Format: value: newValue; value2: newValue2. Multiple values map to one value is also supported: value,value2,value3: 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. Read more about this in a dedicated section below.
tg-edge	Optional	cover	When should the calculation starts and ends. cover means off-screen to off-screen, the calculation begins right on the element appear at the bottom, ends after the element disappears at the top; inset means 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 calculation result from another element instead. The value of tg-follow is the value of the target element's tg-ref. Important: When tg-follow is set, tg-from, tg-to, tg-steps, tg-step and tg-edge will be ignored in the same element.
tg-ref	Optional	(Empty)	Define the name that can be referenced (followed) by another element using tg-follow.

Value Mapping

Numbers don't always work. For example, we want to update the text color based on the scroll value, the tg-map attribute 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 let 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 define the exact value of increment, please note that if tg-step is defined, tg-steps will be ingored.

Noise Reduction

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

<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. Let's use 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 blue when it arrives at the middle of the page and never turn black again because there is nothing to trigger 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>

This works, but the code becomes redundant quickly. To solve this, we can switch the filter mode to exact by adding an ! at the end 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 other than 5.

The design by adding an ! directly to tg-filter is that this requirement should only happen when tg-filter is used. Separating to yet another attribute should not be necessary and might cause misunderstanding.

Value Inheritance

Just like some CSS properties, the values of tg- attributes (except tg-follow, tg-ref) are inherited from parents if not being set in the current element. If we do not want to inherit 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 rooms for scrolling */
  }

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

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

Customising the Prefix

In case you are concerning about the tg- prefix that is not 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 customised 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.