Why don't variables work normally with Lit's `css` template literal function?

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05-10-2024

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Why Lit's css Template Literal Doesn't Play Nice with Variables

Lit, a popular library for building web components, offers a handy css template literal function to style your components. But if you're used to the way variables work in regular JavaScript, you might find yourself scratching your head when trying to use them within Lit's css function.

Let's break down why this happens and how to get around it.

The Scenario:

Imagine you're creating a component that needs to dynamically adjust its color based on a user's preference. You might write something like this:

import { LitElement, html, css } from 'lit';
import { customElement, property } from 'lit/decorators.js';

@customElement('my-component')
export class MyComponent extends LitElement {
  @property({ type: String }) color = 'blue'; 

  static styles = css`
    .container {
      background-color: ${this.color}; /* This won't work as expected */
    }
  `;

  render() {
    return html`
      <div class="container">
        My component content
      </div>
    `;
  }
}

You expect the container's background color to be dynamically set to the value of the color property, but instead, you get an error.

The Explanation:

The problem lies in the way Lit handles the css template literal function. It doesn't allow you to directly use JavaScript variables within it. This is because the CSS code is processed at compile time, while your variables are evaluated at runtime.

The Solution:

To work around this limitation, you can use CSS variables:

import { LitElement, html, css } from 'lit';
import { customElement, property } from 'lit/decorators.js';

@customElement('my-component')
export class MyComponent extends LitElement {
  @property({ type: String }) color = 'blue'; 

  static styles = css`
    :host {
      --my-color: ${this.color};
    }

    .container {
      background-color: var(--my-color);
    }
  `;

  render() {
    return html`
      <div class="container">
        My component content
      </div>
    `;
  }
}

Here's how this works:

1. Define a CSS Variable: We declare a CSS variable named --my-color within the :host selector, which ensures that the variable is applied at the component level.
2. Assign the Variable: We assign the value of the color property to the --my-color variable using the template literal syntax.
3. Use the Variable: Finally, we use the var(--my-color) function in the background-color property of the .container class.

This way, Lit can compile the CSS code with the CSS variable declaration, and the JavaScript runtime will update the variable value when needed, dynamically changing the background color.

In Conclusion:

Understanding the difference between compile-time and runtime execution is crucial when working with Lit's css function. By leveraging CSS variables, we can seamlessly incorporate dynamic styling into our web components.

This approach allows for a more flexible and maintainable way of managing styles, providing the power of JavaScript variables within the confines of CSS.