make npm helper windows aware

src/docgen/view/_npm.ts

@@ -132,17 +132,20 @@ export class Npm {
       return this.#npmCommand;
     }
 
+    // Get the platform specific npm command
+    const npm = npmPlatformAwareCommand();
+
     try {
       // If the npm in $PATH is >= v7, we can use that directly. The
       // `npm version --json` command returns a JSON object containing the
       // versions of several components (npm, node, v8, etc...). We are only
       // interested in the `npm` key here.
       const { exitCode, stdout } = await this.runCommand(
-        'npm', ['version', '--json'],
+        npm, ['version', '--json'],
         chunksToObject,
       );
       if (exitCode === 0 && major((stdout as any).npm) >= 7) {
-        return this.#npmCommand = 'npm';
+        return this.#npmCommand = npm;
       }
     } catch (e) {
       this.logger('Could not determine version of npm in $PATH:', e);
@@ -152,7 +155,7 @@ export class Npm {
     // the full type system.
     this.logger('The npm in $PATH is not >= v7. Installing npm@8 locally...');
     const result = await this.runCommand(
-      'npm',
+      npm,
       ['install', 'npm@8', '--no-package-lock', '--no-save', '--json'],
       chunksToObject,
       {
@@ -162,7 +165,7 @@ export class Npm {
     );
     assertSuccess(result);
 
-    this.#npmCommand = join(this.workingDirectory, 'node_modules', '.bin', 'npm');
+    this.#npmCommand = join(this.workingDirectory, 'node_modules', '.bin', npm);
     this.logger(`Done installing npm@8 at ${this.#npmCommand}`);
     return this.#npmCommand;
   }
@@ -330,3 +333,15 @@ type ResponseObject =
   | { readonly error: { readonly code: string; readonly summary: string; readonly detail: string } }
   // The successful objects are treated as opaque blobs here
   | { readonly error: undefined; readonly [key: string]: unknown };
+
+/**
+ * Get the npm binary path depending on the platform.
+ * @returns "npm.exe" on Windows, otherwise "npm"
+ */
+function npmPlatformAwareCommand() {
+  if (process.platform === 'win32') {
+    return 'npm.cmd';
+  }
+
+  return 'npm';
+}