Nuxt/packages/kit/src/module/install.ts

import type { LegacyNuxtModule, NuxtModule, ModuleMeta, ModuleInstallOptions, ModuleOptions, ModuleSrc, Nuxt } from '@nuxt/schema'
import { resolveModule, requireModule, importModule } from '../internal/cjs'
import { resolveAlias } from '../resolve'
import { defineNuxtModule } from './define'
import { createModuleContainer } from './container'

/** Installs a module on a Nuxt instance. */
export async function installModule (nuxt: Nuxt, installOpts: ModuleInstallOptions) {
  let src: ModuleSrc
  let options: ModuleOptions = {}
  const meta: ModuleMeta = {}

  // Extract src, meta and options
  if (typeof installOpts === 'string') {
    src = installOpts
  } else if (Array.isArray(installOpts)) {
    [src, options] = installOpts
  } else if (typeof installOpts === 'object') {
    if (installOpts.src || installOpts.handler) {
      src = installOpts.src || installOpts.handler
      options = installOpts.options
      Object.assign(meta, installOpts.meta)
    } else {
      src = installOpts as NuxtModule
    }
  } else {
    src = installOpts
  }

  // Resolve as legacy handler
  let handler: LegacyNuxtModule
  if (typeof src === 'string') {
    const _src = resolveModule(resolveAlias(src, nuxt.options.alias), { paths: nuxt.options.modulesDir })
    // TODO: also check with type: 'module' in closest `package.json`
    const isESM = _src.endsWith('.mjs') || meta.isESM
    handler = isESM ? await importModule(_src) : requireModule(_src)
    if (!meta.name) {
      meta.name = src
    }
  } else if (typeof src === 'function') {
    handler = src
  } else {
    handler = defineNuxtModule(src)
  }

  // Merge meta
  if (handler.meta) {
    Object.assign(meta, handler.meta)
  }

  // Ensure module is required once
  if (typeof meta.name === 'string') {
    nuxt.options._requiredModules = nuxt.options._requiredModules || {}
    if (nuxt.options._requiredModules[meta.name]) {
      return
    }
    nuxt.options._requiredModules[meta.name] = true
  }

  // Execute in legacy container
  const container = createModuleContainer(nuxt)
  await handler.call(container, options)
}