Travis build: 840 [ci skip]

README.md

@@ -133,6 +133,7 @@
 * [`defer`](#defer)
 * [`functionName`](#functionname)
 * [`memoize`](#memoize)
+* [`once`](#once)
 * [`runPromisesInSeries`](#runpromisesinseries)
 * [`sleep`](#sleep)
 
@@ -2458,6 +2459,34 @@ anagramsCached('javascript'); // returns virtually instantly since it's now cach
 <br>[⬆ Back to top](#table-of-contents)
 
 
+### once
+
+Ensures a function is called only once.
+
+Utilizing a closure, use a flag, `called`, and set it to `true` once the function is called for the first time, preventing it from being called again. 
+Allow the function to be supplied with an arbitrary number of arguments using the spread (`...`) operator.
+
+```js
+const once = fn =>
+  (called => (...args) => (!called ? ((called = true), fn(...args)) : undefined))();
+```
+
+<details>
+<summary>Examples</summary>
+
+```js
+const startApp = event => {
+  // initializes the app
+  console.log(event); // access to any arguments supplied
+};
+document.addEventListener('click', once(startApp)); // only runs `startApp` once upon click
+```
+
+</details>
+
+<br>[⬆ Back to top](#table-of-contents)
+
+
 ### runPromisesInSeries
 
 Runs an array of promises in series.

snippets/once.md

@@ -6,7 +6,8 @@ Utilizing a closure, use a flag, `called`, and set it to `true` once the functio
 Allow the function to be supplied with an arbitrary number of arguments using the spread (`...`) operator.
 
 ```js
-const once = fn => (called => (...args) => !called ? (called = true, fn(...args)) : undefined)()
+const once = fn =>
+  (called => (...args) => (!called ? ((called = true), fn(...args)) : undefined))();
 ```
 
 ```js