diff --git a/src/index.ts b/src/index.ts
index 585754a..a9f6120 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -9,15 +9,14 @@ export class Timeout {
     private _startedAt: number;
     private _callback: () => void;
     private _timeLeft: number;
-    private _timerId: number;
+    private _timerId: any; // not possible in any other way
 
     public constructor(callback: () => void, timeMS: number, autoStart?: boolean) {
-        var self = this;
-        this._callback = function () { self.state = 3; callback(); }
+        this._callback = () => { this.state = 3; callback(); }
         this.currentTime = timeMS;
         this._startedAt = 0;
         this._timeLeft = timeMS;
-        this._timerId = -1;
+        this._timerId = null;
         this.state = 0;
         if (autoStart) this.start();
     }
@@ -27,15 +26,15 @@ export class Timeout {
     public start(timeMS?: number): this {
         if (this.state == 3) this.stop();
         if (arguments.length > 0 && this.state == 0) this.currentTime = timeMS;
-        if (this._timerId == -1) {
+        if (this._timerId == null) {
             if (this.state == 2) {
                 this._startedAt = Date.now();
-                this._timerId = <any>setTimeout(this._callback, this._timeLeft);
+                this._timerId = setTimeout(this._callback, this._timeLeft);
                 this.state = 1;
             }
             else if (this.state == 0) {
                 this._startedAt = Date.now();
-                this._timerId = <any>setTimeout(this._callback, this.currentTime);
+                this._timerId = setTimeout(this._callback, this.currentTime);
                 this.state = 1;
             }
         }
@@ -44,16 +43,16 @@ export class Timeout {
 
     public pause(): this {
         if (this.state != 1) return this;
-        clearTimeout(<any>this._timerId);
-        this._timerId = -1;
+        clearTimeout(this._timerId);
+        this._timerId = null;
         this._timeLeft -= Date.now() - this._startedAt;
         this.state = 2;
         return this;
     }
 
     public stop(): this {
-        clearTimeout(<any>this._timerId);
-        this._timerId = -1;
+        clearTimeout(this._timerId);
+        this._timerId = null;
         this._timeLeft = 0;
         this.state = 0;
         return this;
@@ -71,16 +70,15 @@ export class Interval {
     private _lastTrigger: number;
     private _callback: () => void;
     private _timeLeft: number;
-    private _timerId: number;
+    private _timerId: any; // not possible in any other way
     private _isInTimeout: boolean;
 
     public constructor(callback: (() => void), timeMS?: number, autoStart?: boolean) {
-        var self = this;
         this._callback = () => { this._lastTrigger = Date.now(); callback(); };
         this.currentTime = timeMS;
         this._lastTrigger = 0;
         this._timeLeft = timeMS;
-        this._timerId = -1;
+        this._timerId = null;
         this.state = 0;
         this._isInTimeout = false;
         if (autoStart) this.start();
@@ -90,20 +88,19 @@ export class Interval {
     public start(timeMS: number): this;
     public start(timeMS?: number): this {
         if (arguments.length > 0 && this.state == 0) this.currentTime = timeMS;
-        if (this._timerId == -1) {
+        if (this._timerId == null) {
             if (this.state == 2) {
                 this._lastTrigger = Date.now();
-                var self = this;
-                this._timerId = <any>setTimeout(function () {
+                this._timerId = setTimeout(() => {
                     this._isInTimeout = false;
-                    this._timerId = setInterval(self._callback, self.currentTime);
+                    this._timerId = setInterval(this._callback, this.currentTime);
                 }, this._timeLeft);
                 this.state = 1;
                 this._isInTimeout = true;
             }
             else if (this.state == 0) {
                 this._lastTrigger = Date.now();
-                this._timerId = <any>setInterval(this._callback, this.currentTime);
+                this._timerId = setInterval(this._callback, this.currentTime);
                 this.state = 1;
             }
         }
@@ -112,16 +109,16 @@ export class Interval {
 
     public pause(): this {
         if (this.state != 1) return this;
-        this._isInTimeout ? clearTimeout(<any>this._timerId) : clearInterval(<any>this._timerId);
-        this._timerId = -1;
+        this._isInTimeout ? clearTimeout(this._timerId) : clearInterval(this._timerId);
+        this._timerId = null;
         this._timeLeft -= Date.now() - this._lastTrigger;
         this.state = 2;
         return this;
     }
 
     public stop(): this {
-        this._isInTimeout ? clearTimeout(<any>this._timerId) : clearInterval(<any>this._timerId);
-        this._timerId = -1;
+        this._isInTimeout ? clearTimeout(this._timerId) : clearInterval(this._timerId);
+        this._timerId = null;
         this._timeLeft = 0;
         this.state = 0;
         this._isInTimeout = false;