diff --git a/types/toobusy-js/index.d.ts b/types/toobusy-js/index.d.ts
index 2d7f2688a7..2e6aca6c4b 100644
--- a/types/toobusy-js/index.d.ts
+++ b/types/toobusy-js/index.d.ts
@@ -1,19 +1,66 @@
 // Type definitions for toobusy-js 0.5
 // Project: https://github.com/STRML/node-toobusy
 // Definitions by: Arne Schubert <https://github.com/atd-schubert>
+//                 BendingBender <https://github.com/BendingBender>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 export = toobusy_js;
 
+/**
+ * Main export function.
+ * @return True if node process is too busy.
+ */
 declare function toobusy_js(): boolean;
 
 declare namespace toobusy_js {
-    function interval(newInterval: number): number;
+    /**
+     * Sets or gets the current check interval.
+     * If you want more sensitive checking, set a faster (lower) interval. A lower maxLag can also create a more
+     * sensitive check.
+     * @param  [newInterval] New interval to set. If not provided, will return the existing interval.
+     * @return               New or existing interval.
+     */
+    function interval(newInterval?: number): number;
+    /**
+     * Returns last lag reading from last check interval.
+     * @return Lag in ms.
+     */
     function lag(): number;
-    function maxLag(newLag: number): number;
-    function smoothingFactor(newFactor: number): number;
+    /**
+     * Set or get the current max latency threshold. Default is 70ms.
+     *
+     * Note that if event loop lag goes over this threshold, the process is not always 'too busy' - the farther
+     * it goes over the threshold, the more likely the process will be considered too busy.
+     *
+     * The percentage is equal to the percent over the max lag threshold. So 1.25x over the maxLag will indicate
+     * too busy 25% of the time. 2x over the maxLag threshold will indicate too busy 100% of the time.
+     * @param  [newLag] New maxLag (highwater) threshold.
+     * @return          New or existing maxLag (highwater) threshold.
+     */
+    function maxLag(newLag?: number): number;
+    /**
+     * Set or get the smoothing factor. Default is 0.3333....
+     *
+     * The smoothing factor per the standard exponential smoothing formula "αtn + (1-α)tn-1"
+     * See: https://en.wikipedia.org/wiki/Exponential_smoothing
+     *
+     * @param  [newFactor] New smoothing factor.
+     * @return             New or existing smoothing factor.
+     */
+    function smoothingFactor(newFactor?: number): number;
+    /**
+     * Shuts down toobusy.
+     *
+     * Not necessary to call this manually, only do this if you know what you're doing. `unref()` is called
+     * on toobusy's check interval, so it will never keep the server open.
+     */
     function shutdown(): void;
+    /**
+     * Registers an event listener for lag events,
+     * optionally specify a minimum value threshold for events being emitted
+     * @param fn                 Function of form onLag(value: number) => void
+     * @param [threshold=maxLag] Optional minimum lag value for events to be emitted
+     */
     function onLag(fn: (lag: number) => void, threshold?: number): void;
-
     function started(): boolean;
 }
diff --git a/types/toobusy-js/toobusy-js-tests.ts b/types/toobusy-js/toobusy-js-tests.ts
index 121b84ddb4..2678dcc777 100644
--- a/types/toobusy-js/toobusy-js-tests.ts
+++ b/types/toobusy-js/toobusy-js-tests.ts
@@ -1,4 +1,4 @@
-import toobusy = require("toobusy-js");
+import toobusy = require('toobusy-js');
 
 let numberValue = 1;
 let booleanValue = true;
@@ -7,11 +7,19 @@ booleanValue = toobusy();
 booleanValue = toobusy.started();
 
 numberValue = toobusy.interval(numberValue);
+numberValue = toobusy.interval();
 numberValue = toobusy.lag();
 numberValue = toobusy.maxLag(numberValue);
+numberValue = toobusy.maxLag();
 numberValue = toobusy.smoothingFactor(numberValue);
-
-toobusy.onLag((duration: number) => {});
-toobusy.onLag((duration: number) => {}, numberValue);
-
+numberValue = toobusy.smoothingFactor();
 toobusy.shutdown();
+
+toobusy.onLag(duration => {
+    // $ExpectType number
+    duration;
+});
+toobusy.onLag(duration => {
+    // $ExpectType number
+    duration;
+}, numberValue);