Function withAlpha

packages/tailwindcss/src/utilities.ts:176–193

(value: string, alpha: string)

 * Apply opacity to a color using `color-mix`.
 */
export function withAlpha(value: string, alpha: string): string {
  if (alpha === null) return value

  // Convert numeric values (like `0.5`) to percentages (like `50%`) so they
  // work properly with `color-mix`. Assume anything that isn't a number is
  // safe to pass through as-is, like `var(--my-opacity)`.
  let alphaAsNumber = Number(alpha)
  if (!Number.isNaN(alphaAsNumber)) {
    alpha = `${alphaAsNumber * 100}%`
  }

  // No need for `color-mix` if the alpha is `100%`
  if (alpha === '100%') {
    return value
  }

  return `color-mix(in oklab, ${value} ${alpha}, transparent)`
}

/**
 * Replace the opacity in a color using relative color syntax.