Improve the postinstall hook (#3933)

Author: DmitrySharabin

.eslintignore

@@ -0,0 +1,2 @@
+# In the script, we use syntax that this version of ESLint can't parse
+postinstall.js

package.json

@@ -26,7 +26,7 @@
 		"test:runner": "mocha tests/testrunner-tests.js",
 		"test": "npm-run-all test:*",
 		"release": "release-it",
-		"postinstall": "npm run build"
+		"postinstall": "node postinstall.js"
 	},
 	"repository": {
 		"type": "git",

postinstall.js

@@ -0,0 +1,27 @@
+// The script is compatible with Node.js 10+
+
+const { execSync } = require('child_process');
+const { resolve } = require('path');
+
+function gulpInstalled () {
+	try {
+		require.resolve('gulp');
+		return true;
+	}
+	catch {
+		return false;
+	}
+}
+
+const root = resolve(__dirname, '.');
+
+if (!gulpInstalled()) {
+	console.log('[postinstall] Dependencies missing — installing...');
+	execSync('npm install', { cwd: root, stdio: 'inherit' });
+}
+else {
+	console.log('[postinstall] Dependencies already installed.');
+}
+
+console.log('[postinstall] Running build...');
+execSync('npm run build', { cwd: root, stdio: 'inherit' });