fix: lib/helpers/install.ts to better support pnpm and properly respect root argument (#64418)

This fixes and improves the `lib/helpers/install.ts` file (`install()`)
method to better support pnpm and properly respect the `root` argument.

I encountered an issue where by running `next <command> <project-dir>`,
and the `<command>` involved installing missing dependencies (like
`lint` or my upcoming `experimental-test`), it was not executing pnpm in
my `<project-dir>` because pnpm doesn't have a `--cwd` flag.



Closes NEXT-3092

Co-authored-by: Zack Tanner <[email protected]>

Author: Ethan-Arrowood

packages/next/src/lib/helpers/install.ts

@@ -1,15 +1,14 @@
 import { yellow } from '../picocolors'
 import spawn from 'next/dist/compiled/cross-spawn'
-
-export type PackageManager = 'npm' | 'pnpm' | 'yarn'
+import type { PackageManager } from './get-pkg-manager'
 
 interface InstallArgs {
   /**
-   * Indicate whether to install packages using npm, pnpm or Yarn.
+   * Indicate whether to install packages using npm, pnpm, or yarn.
    */
   packageManager: PackageManager
   /**
-   * Indicate whether there is an active Internet connection.
+   * Indicate whether there is an active internet connection.
    */
   isOnline: boolean
   /**
@@ -19,81 +18,52 @@ interface InstallArgs {
 }
 
 /**
- * Spawn a package manager installation with either Yarn or NPM.
+ * Spawn a package manager installation with either npm, pnpm, or yarn.
  *
  * @returns A Promise that resolves once the installation is finished.
  */
 export function install(
   root: string,
-  dependencies: string[] | null,
+  dependencies: string[],
   { packageManager, isOnline, devDependencies }: InstallArgs
 ): Promise<void> {
-  /**
-   * (p)npm-specific command-line flags.
-   */
-  const npmFlags: string[] = []
-  /**
-   * Yarn-specific command-line flags.
-   */
-  const yarnFlags: string[] = []
-  /**
-   * Return a Promise that resolves once the installation is finished.
-   */
-  return new Promise((resolve, reject) => {
-    let args: string[]
-    let command = packageManager
-    const useYarn = packageManager === 'yarn'
+  let args: string[] = []
 
-    if (dependencies && dependencies.length) {
-      /**
-       * If there are dependencies, run a variation of `{packageManager} add`.
-       */
-      if (useYarn) {
-        /**
-         * Call `yarn add --exact (--offline)? (-D)? ...`.
-         */
-        args = ['add', '--exact']
-        if (!isOnline) args.push('--offline')
-        args.push('--cwd', root)
-        if (devDependencies) args.push('--dev')
-        args.push(...dependencies)
-      } else {
-        /**
-         * Call `(p)npm install [--save|--save-dev] ...`.
-         */
-        args = ['install', '--save-exact']
-        args.push(devDependencies ? '--save-dev' : '--save')
-        args.push(...dependencies)
-      }
+  if (dependencies.length > 0) {
+    if (packageManager === 'yarn') {
+      args = ['add', '--exact']
+      if (devDependencies) args.push('--dev')
+    } else if (packageManager === 'pnpm') {
+      args = ['add', '--save-exact']
+      args.push(devDependencies ? '--save-dev' : '--save-prod')
     } else {
-      /**
-       * If there are no dependencies, run a variation of `{packageManager}
-       * install`.
-       */
-      args = ['install']
-      if (!isOnline) {
-        console.log(yellow('You appear to be offline.'))
-        if (useYarn) {
-          console.log(yellow('Falling back to the local Yarn cache.'))
-          console.log()
-          args.push('--offline')
-        } else {
-          console.log()
-        }
-      }
+      // npm
+      args = ['install', '--save-exact']
+      args.push(devDependencies ? '--save-dev' : '--save')
     }
-    /**
-     * Add any package manager-specific flags.
-     */
-    if (useYarn) {
-      args.push(...yarnFlags)
-    } else {
-      args.push(...npmFlags)
+
+    args.push(...dependencies)
+  } else {
+    args = ['install'] // npm, pnpm, and yarn all support `install`
+
+    if (!isOnline) {
+      args.push('--offline')
+      console.log(yellow('You appear to be offline.'))
+      if (packageManager !== 'npm') {
+        console.log(
+          yellow(`Falling back to the local ${packageManager} cache.`)
+        )
+      }
+      console.log()
     }
+  }
+
+  return new Promise((resolve, reject) => {
     /**
      * Spawn the installation process.
      */
-    const child = spawn(command, args, {
+    const child = spawn(packageManager, args, {
+      cwd: root,
       stdio: 'inherit',
       env: {
         ...process.env,
@@ -106,7 +76,7 @@ export function install(
     })
     child.on('close', (code) => {
       if (code !== 0) {
-        reject({ command: `${command} ${args.join(' ')}` })
+        reject({ command: `${packageManager} ${args.join(' ')}` })
         return
       }
       resolve()