Next.js Codebase Analysis <> create-next-app <> index.ts explained - Part 1.18

Finally, I am past the prompts you see when you enter npx create-next-app. In this article, I will talk about the a code chunk from createApp function.

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try {
    await createApp({
      appPath: resolvedProjectPath,
      packageManager,
      example: example && example !== 'default' ? example : undefined,
      examplePath: program.examplePath,
      typescript: program.typescript,
      tailwind: program.tailwind,
      eslint: program.eslint,
      appRouter: program.app,
      srcDir: program.srcDir,
      importAlias: program.importAlias,
    })
  }

My first thought is that program variable has been updated with the responses you provide and is further used in createApp function as an argument.

I like the fact that it is a JSON object as an argument, a good thumb rule from clean code book is that your function can at most have 3 arguments, if you need more than 3 arguments, you might want to break down your function further or use an object like in this case.

createApp function:

createApp is an async function export from create-app.ts

export async function createApp({
  appPath,
  packageManager,
  example,
  examplePath,
  typescript,
  tailwind,
  eslint,
  appRouter,
  srcDir,
  importAlias,
}: { 
  // this can be defined as type in the same file at the top 
  // But that's fine assuming this is only the place it is used.
  appPath: string
  packageManager: PackageManager
  example?: string
  examplePath?: string
  typescript: boolean
  tailwind: boolean
  eslint: boolean
  appRouter: boolean
  srcDir: boolean
  importAlias: string
}): Promise<void> {
  let repoInfo: RepoInfo | undefined
  // there is mode derived from typescript flag
  const mode: TemplateMode = typescript ? 'ts' : 'js'
  // nested ternary? nice.
  // so there is two kinds of templates,
  // one with router and the other without
  const template: TemplateType = appRouter
    ? tailwind
      ? 'app-tw'
      : 'app'
    : tailwind
    ? 'default-tw'
    : 'default'

Conclusion:

I see that the argument type is defined next to argument without any explicit declaration of type somewhere at the top of the file, I would be okay with this assuming this type is not required else where in the codebase, otherwise I would write a type for this argument at the top of file and export it for reusability purposes.

Nested ternary, I don’t use that style much, but I can use it if the need be. I completely forgot we could do such a nested ternary, quite the reminder.

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