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.

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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.

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:

<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 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.

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>

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.

Just like some CSS properties, the values of tg- attributes are inherit 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>

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>

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.

Feel free to fork this repository and submit pull requests. Discussions will be in GitHub Issues.

Trigger.js is MIT Licensed.