@stylexjs/shared

This package contains most of the core JavaScript logic for stylex.

It exports two primary functions create and keyframes.

1. create - takes a map of style rules. The return value includes: a) the map with each style value replaced by a unique, reproducible, hashed className string, and b) a list of the CSS styles to be inserted into the document.
2. keyframes - takes a @keyframes animation as JS object. Returns a hashed string and the style to be injected.

⭐️ create

The stylex.create function is implemented here and can be found within stylex-create.js and is the default export of a function named styleXCreateSet(...).

styleXCreateSet(...)

The function is called styleXCreateSet because stylex.create transforms a "set" or collection of multiple style namespaces

This function itself mostly just traverses over the objects to run each namespaces through the styleXCreateNamespace(...) function. Other than that, it takes the styles to be injected from each namespace in a Namespace Set and deduplicates them so the style isn't injected multiple times if it's used within multiple Namespaces in the same set.

styleXCreateNamespace(...)

This function has been kept separate in case we want to add a new function to the StyleX API in the future called stylex.createOne which transforms a single namespace instead of a Namespace Set

This function is responsible to transforming a namespace to a Compiled Namespace by hashing each key value pair and returning an object where the values have been replaced by classNames.

Step 1

The first step here is expanding all shorthands into their individual properties. To do this we .flatMap over the object entries of the Namespace and use the expandShorthands(...) function defined within expand-shorthands.js

Step 2

We hash each style [key, value] pair and generate a className and an associated CSS rule. This is done in the convertToClassName(...) function defined within convert-to-className.js. (Explained below)

Step 3

Using the classNames generated in step 2 above, we collect all the individual style keys along with their associated classNames in the resolvedNamespace object.

All the generated CSS rules from step 2 are collected in the injectedStyles object.

The [resolvedNamespace, injectedStyles] is returned.

Back to styleXCreateSet(...)

styleXCreateSet(...) takes all the [resolvedNamespace, finalInjectedStyles] tuples and returns a tuple of [compiledNamespaceSet, allInjectedStyles]

Back to create with the @stylexjs/babel-plugin package

The create function within the babel plugin package takes the stylex.create(...) function call and replaces it with the compiledNamespaceSet.

It also takes each of the injectedStyles and:

1. Either injects it as a stylex.inject call (if in dev mode)
2. Or, adds it to the array of injected styles on babel.state.metadata

⭐️ keyframes

This is the function backing stylex.keyframes. It works similarly to create but it's more simplified since it only defines a single CSS @keyframes rule and returns a single string.

Here again, the source AST is converted to a JS object and passed to stylex-keyframes.js within the shared package.

There, first the shorthands are expanded and then the whole objects is hashed. The resulting hash is used as the generated animation name for a @keyframes rule.

The "name" and the CSS @keyframes rules are returned as a tuple.

The stylex.keyframes call is replaced with the final string.

The CSS @keyframes rule is either injected using stylex.inject in dev mode or set onto the stylex array on babel.state.metadata.

convert-to-className (shared package)

This function is responsible for converting a single style key-value pair into a tuple of [key, className, CSSRules]

It does so in the following steps:

1. Convert the camelCased keys that are used by end-users to define Namespaces and convert them to the dash-separated keys used within CSS.
2. Hash key + (any pseudo or at-rule) + value to generate a className
3. Generate the CSS rule using the generateCSSRule function defined in generate-css-rule.js in the shared package.

generate-css-rule.js

This function takes a CSS key value pair, checks if has an RTL counterpart and returns them along side a pre-configured priority based on the type of CSS rule it is.