diff --git a/bin/glob-tsc.js b/bin/glob-tsc.js
index d8697ff..b37687d 100644
--- a/bin/glob-tsc.js
+++ b/bin/glob-tsc.js
@@ -6,7 +6,11 @@ var spawn = require('cross-spawn'),
     commandArgs = options.unknown.concat(helper.resolveTSFiles()),
     proc = spawn(helper.getTSCCommand(), commandArgs, { stdio: 'inherit' });
 
-proc.on('exit', function (code, signal) {
+/**
+ * @param {number} code
+ * @param {number} signal
+ */
+function onExit (code, signal) {
     process.on('exit', function(){
         if (signal) {
             process.kill(process.pid, signal);
@@ -14,7 +18,9 @@ proc.on('exit', function (code, signal) {
             process.exit(code);
         }
     });
-});
+}
+
+proc.on('exit', onExit);
 
 // terminate children.
 process.on('SIGINT', function () {
diff --git a/lib/program-helper.js b/lib/program-helper.js
index f32432e..619ffce 100644
--- a/lib/program-helper.js
+++ b/lib/program-helper.js
@@ -20,11 +20,13 @@ function error() {
 
 /**
  * Resolve file globs based in command line or tsconfig.json file.
- * @returns {Array}
+ * May throw an error.
+ * @returns {string[]}
  */
 function resolveGlobs() {
     var options = Helper.getOptions(),
         tsConfigFile,
+        /** @type{string[]} */
         tsConfig;
 
     if (options['filesGlob'] && options['filesGlob'].length) {
@@ -41,14 +43,15 @@ function resolveGlobs() {
 
             return tsConfig['filesGlob'];
         } catch (err) {
-            error('tsconfig.json file is not accessible.', err);
+            // explicitly throw here to pass --strictNullChecks
+            throw error('tsconfig.json file is not accessible.', err);
         }
     }
 }
 
 /**
  * Check if a file exists
- * @param cmdPath
+ * @param {string} cmdPath
  * @returns {boolean}
  */
 function fileExists(cmdPath) {
@@ -80,9 +83,10 @@ Helper.getTSCCommand = function () {
 /**
  * Resolve ts file paths
  *
- * @returns {Array}
+ * @returns {string[]}
  */
 Helper.resolveTSFiles = function () {
+    /** @type{string[]} */
     var files = [];
 
     resolveGlobs().forEach(function (pattern) {
@@ -114,4 +118,7 @@ Helper.getOptions = function () {
     return commander;
 };
 
-module.exports = Helper;
\ No newline at end of file
+/**
+ * @typedef {object} Helper
+ */
+module.exports = Helper;
diff --git a/package.json b/package.json
index b923753..617d650 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,7 @@
     "glob-tsc": "./bin/glob-tsc.js"
   },
   "scripts": {
-    "checkjs": "node bin/glob-tsc.js --allowJs --checkJs --noEmit --resolveJsonModule --target es5 -g bin/**/*.js,test/**/*.js",
+    "checkjs": "node bin/glob-tsc.js --allowJs --checkJs --noEmit --resolveJsonModule --target es5 --strictFunctionTypes --strictBindCallApply --noImplicitThis --strictNullChecks --noImplicitAny -g bin/**/*.js,test/**/*.js",
     "test": "npm-run-all checkjs test:mocha",
     "test:mocha": "mocha --timeout 5000",
     "test:watch": "_mocha --watch"
@@ -28,7 +28,11 @@
   },
   "license": "ISC",
   "devDependencies": {
+    "@types/chai": "^4.2.4",
+    "@types/cross-spawn": "^6.0.1",
+    "@types/glob": "^7.1.1",
     "@types/mocha": "^5.2.7",
+    "@types/mock-require": "^2.0.0",
     "@types/node": "^12.11.7",
     "chai": "^4.2.0",
     "mocha": "^6.2.2",
diff --git a/test/cross-spawn.mock.js b/test/cross-spawn.mock.js
index c6dc556..147c856 100644
--- a/test/cross-spawn.mock.js
+++ b/test/cross-spawn.mock.js
@@ -1,14 +1,28 @@
 var expect = require('chai').expect;
 
+/**
+ * @constructor
+ */
 var Process = function () {
+    /** @type {Record<string, Function>} */
     var listeners = {};
 
-    this.on = function (event, cb) {
+    /**
+     * @param {string} event
+     * @param {Function} cb
+     */
+    function onEvent (event, cb) {
         listeners[event] = cb;
     };
+
+    this.on = onEvent;
 };
 
 module.exports =
+    /**
+     * @param {string} cmd
+     * @param {string[]} args
+     */
     function (cmd, args) {
         expect(cmd).to.equal('tsc');
         expect(args).to.eql(['src/User.ts']);