49 lines
2.0 KiB
Markdown
49 lines
2.0 KiB
Markdown
---
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title: debouncePromise
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tags: function,promise,advanced
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---
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Creates a debounced function that returns a promise, but delays invoking the provided function until at least `ms` milliseconds have elapsed since the last time it was invoked.
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All promises returned during this time will return the same data.
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- Each time the debounced function is invoked, clear the current pending timeout with `clearTimeout()` and use `setTimeout()` to create a new timeout that delays invoking the function until at least `ms` milliseconds has elapsed.
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- Use `Function.prototype.apply()` to apply the `this` context to the function and provide the necessary arguments.
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- Create a new `Promise` and add its `resolve` and `reject` callbacks to the `pending` promises stack.
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- When `setTimeout` is called, copy the current stack (as it can change between the provided function call and its resolution), clear it and call the provided function.
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- When the provided function resolves/rejects, resolve/reject all promises in the stack (copied when the function was called) with the returned data.
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- Omit the second argument, `ms`, to set the timeout at a default of `0` ms.
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```js
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const debouncePromise = (fn, ms = 0) => {
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let timeoutId;
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const pending = [];
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return (...args) =>
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new Promise((res, rej) => {
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clearTimeout(timeoutId);
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timeoutId = setTimeout(() => {
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const currentPending = [...pending];
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pending.length = 0;
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Promise.resolve(fn.apply(this, args)).then(
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data => {
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currentPending.forEach(({ resolve }) => resolve(data));
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},
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error => {
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currentPending.forEach(({ reject }) => reject(error));
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}
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);
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}, ms);
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pending.push({ resolve: res, reject: rej });
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});
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};
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```
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```js
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const fn = arg => new Promise(resolve => {
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setTimeout(resolve, 1000, ['resolved', arg]);
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});
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const debounced = debouncePromise(fn, 200);
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debounced('foo').then(console.log);
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debounced('bar').then(console.log);
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// Will log ['resolved', 'bar'] both times
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```
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