In this article, we analyze buildDesignSystem function in Tailwind CSS source code.

DesignSystem type picked from design-system.ts

export type DesignSystem = {
  theme: Theme
  utilities: Utilities
  variants: Variants

  invalidCandidates: Set

  // Whether to mark utility declarations as !important
  important: boolean

  getClassOrder(classes: string[]): [string, bigint | null][]
  getClassList(): ClassEntry[]
  getVariants(): VariantEntry[]

  parseCandidate(candidate: string): Candidate[]
  parseVariant(variant: string): Variant | null
  compileAstNodes(candidate: Candidate): ReturnType

  getVariantOrder(): Map
  resolveThemeValue(path: string): string | undefined

  // Used by IntelliSense
  candidatesToCss(classes: string[]): (string | null)[]
}

At the time of writing this article, design-system.ts has about 144 LOC.

Let’s discuss how the values returned by DefaultMap utility function is used in designSystem.

let parsedVariants = new DefaultMap((variant) => parseVariant(variant, designSystem))
let parsedCandidates = new DefaultMap((candidate) =>
  Array.from(parseCandidate(candidate, designSystem)),
)
let compiledAstNodes = new DefaultMap((candidate) =>
  compileAstNodes(candidate, designSystem),
)

These variables are used in designSystem object as shown below:

parseCandidate(candidate: string) {
  return parsedCandidates.get(candidate)
},
parseVariant(variant: string) {
  return parsedVariants.get(variant)
},
compileAstNodes(candidate: Candidate) {
  return compiledAstNodes.get(candidate)
},

utilities and variants are the values returned by createUtilities and createVariants.

Keys such as candidatesToCss, getVariantOrder and resolveThemeValue have their function implementations that require furter analysis.

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References:

1. https://github.com/tailwindlabs/tailwindcss/blob/next/packages/tailwindcss/src/design-system.ts

2. https://github.com/tailwindlabs/tailwindcss/blob/c01b8254e822d4f328674357347ca0532f1283a0/packages/tailwindcss/src/index.ts#L319