tidy up install script

install.js

@@ -19,28 +19,23 @@ const path = require('path');
 const child_process = require('child_process');
 const {promisify} = require('util');
 
-const LIB_PATH = path.join(__dirname, 'lib');
-
 const fsAccess = promisify(fs.access);
-
 const exec = promisify(child_process.exec);
 
 const fileExists = async filePath => fsAccess(filePath).then(() => true).catch(() => false);
 
-/**
- * This file is part of public API.
+/*
+ * Now Puppeteer is built with TypeScript, we need to ensure that
+ * locally we have the generated output before trying to install.
  *
- * By default, the `puppeteer` package runs this script during the installation
- * process unless one of the env flags is provided.
- * `puppeteer-core` package doesn't include this step at all. However, it's
- * still possible to install a supported browser using this script when
- * necessary.
+ * For users installing puppeteer this is fine, they will have the
+ * generated lib/ directory as we ship it when we publish to npm.
+ *
+ * However, if you're cloning the repo to contribute, you won't have the
+ * generated lib/ directory so this script checks if we need to run
+ * TypeScript first to ensure the output exists and is in the right
+ * place.
  */
-const supportedProducts = {
-  'chrome': 'Chromium',
-  'firefox': 'Firefox Nightly'
-};
-
 async function compileTypeScript() {
   return exec('npm run tsc').catch(err => {
     console.error('Error running TypeScript', err);
@@ -49,14 +44,29 @@ async function compileTypeScript() {
 }
 
 async function ensureLibDirectoryExists() {
-  const libExists = await fileExists(LIB_PATH);
-
+  const libPath = path.join(__dirname, 'lib');
+  const libExists = await fileExists(libPath);
   if (libExists) return;
 
   logPolitely('Compiling TypeScript before install...');
   await compileTypeScript();
 }
 
+
+/**
+ * This file is part of public API.
+ *
+ * By default, the `puppeteer` package runs this script during the installation
+ * process unless one of the env flags is provided.
+ * `puppeteer-core` package doesn't include this step at all. However, it's
+ * still possible to install a supported browser using this script when
+ * necessary.
+ */
+const supportedProducts = {
+  'chrome': 'Chromium',
+  'firefox': 'Firefox Nightly'
+};
+
 async function download() {
   await ensureLibDirectoryExists();