33 lines
1022 B
Markdown
33 lines
1022 B
Markdown
---
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title: Call function once
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tags: function
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cover: pink-flower-tree
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firstSeen: 2018-01-02T00:40:46+02:00
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lastUpdated: 2020-10-21T21:54:53+03:00
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---
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Ensures a function is called only once.
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- 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.
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- 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.
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- Allow the function to be supplied with an arbitrary number of arguments using the rest/spread (`...`) operator.
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```js
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const once = fn => {
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let called = false;
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return function(...args) {
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if (called) return;
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called = true;
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return fn.apply(this, args);
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};
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};
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```
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```js
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const startApp = function(event) {
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console.log(this, event); // document.body, MouseEvent
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};
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document.body.addEventListener('click', once(startApp));
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// only runs `startApp` once upon click
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```
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