32 lines
997 B
Markdown
32 lines
997 B
Markdown
---
|
|
title: once
|
|
tags: function,intermediate
|
|
firstSeen: 2018-01-02T00:40:46+02:00
|
|
lastUpdated: 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
|
|
```
|