addFunctionSerializer

✋ This is an advanced feature recommended for library authors only.

Typically, if you try to export a function from one of your stylesheets, you’ll get an error that you can only export plain objects, arrays, strings, numbers and null/undefined.

If you’re wanting to create higher level abstractions like Sprinkles or Recipes, this is a big problem!

To address this limitation, the addFunctionSerializer utility allows you to annotate your functions with instructions on how to serialize them.

As a basic example, let’s say we want to create a library called my-awesome-styled-div that lets you create a React component that renders a <div> with static CSS bound to it, and we want our consumers to use it in their .css.ts files like this:

// MyComponent.css.ts

import { styledDiv } from 'my-awesome-styled-div';

export const MyComponent = styledDiv({
  background: 'blue',
  padding: 12
});

Normally if you tried to do this, since MyComponent is a function, it would trigger an error during the build since it can’t be serialized.

Luckily addFunctionSerializer can help us!

To understand how we could make this API work, we’ll first look at what you might expect the implementation of styledDiv to look like.

index.tsx
import React from 'react';
import { style } from '@vanilla-extract/css';

export function styledDiv(styles) {
  const className = style(styles);

  return function Component(
    props: React.ComponentProps<'div'>
  ) {
    return (
      <div
        {...props}
        className={[props.className, className]
          .filter(Boolean)
          .join(' ')}
      />
    );
  };
}

This is a pretty simple implementation, but it has a couple of major problems that prevent it from working. Firstly, there’s no way to convert this function to a string so it can be added to your runtime JavaScript bundle. Secondly, even if it could be serialized somehow, it relies on @vanilla-extract/css which can’t generate static CSS files at runtime.

To fix this, we need to start by splitting the runtime code out into its own module and avoid using @vanilla-extract/css. In this case we’ll create a runtime.tsx file. It must be available as a separate entry point from the library so that it can be imported within runtime code, for example:

import { runtimeStyledDiv } from 'my-awesome-styled-div/runtime';

The runtime implementation would look like this:

runtime.tsx
import React from 'react';

export function runtimeStyledDiv(className) {
  return function Component(
    props: React.ComponentProps<'div'>
  ) {
    return (
      <div
        {...props}
        className={[props.className, className]
          .filter(Boolean)
          .join(' ')}
      />
    );
  };
}

To make this work at runtime, we’ve had to alter the API a bit. Instead of accepting an object of styles, we now accept the generated class name. That’s because this code will run in the end-user’s browser where we no longer have the ability to generate static CSS.

We then need to annotate the generated component within styledDiv using addFunctionSerializer, providing the path to the runtime module, the name of the imported function, and the arguments that should be passed to it.

✋ All arguments passed to the runtime function must be serializable!

index.ts
import { addFunctionSerializer } from '@vanilla-extract/css/functionSerializer';
import { style } from '@vanilla-extract/css';
import { runtimeStyledDiv } from './runtime';

export function styledDiv(styles) {
  const className = style(styles);
  const args = [className];

  // First we call our runtime function at build time
  const Component = runtimeStyledDiv(...args);

  // Then we tell vanilla-extract how to serialize the previous
  // function call by annotating its return value
  addFunctionSerializer(Component, {
    importPath: 'my-awesome-styled-div/runtime',
    importName: 'runtimeStyledDiv',
    args
  });

  // Return the result of calling the runtime function
  return Component;
}

It takes a little bit of wiring to get this working, but thankfully our consumers don’t need to know about any of this! By making use of addFunctionSerializer, we can now break free of the usual constraints around exports and provide much more expressive APIs to our library consumers.

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