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snippets/js/s/once.md

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+---
+title: Call function once
+type: snippet
+language: javascript
+tags: [function]
+cover: pink-flower-tree
+dateModified: 2020-10-21T21:54:53+03:00
+---
+
+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.
+- In order to allow the function to have its `this` context changed (such as in an event listener), the `function` keyword must be used, and the supplied function must have the context applied.
+- Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (`...`) operator.
+
+```js
+const once = fn => {
+  let called = false;
+  return function(...args) {
+    if (called) return;
+    called = true;
+    return fn.apply(this, args);
+  };
+};
+```
+
+```js
+const startApp = function(event) {
+  console.log(this, event); // document.body, MouseEvent
+};
+document.body.addEventListener('click', once(startApp));
+// only runs `startApp` once upon click
+```