Added promiseDebounce

snippets/promiseDebounce.md

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+---
+title: promiseDebounce
+tags: promise,debounce,intermediate
+---
+
+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. All promises returned during this time will return the same data.
+
+- 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.
+- create a new `Promise` and add it's `resolve` and `reject` callback to pending promises stack.
+- When `setTimeout` is called: copy current stack (as it can change between provided function call and resolve), clear it and call provided function.
+- When provided function resolves/rejects, finish all promises from stack (copied when function was called) with returned data.
+- Use `Function.prototype.apply()` to apply the `this` context to the function and provide the necessary arguments.
+- Omit the second argument, `ms`, to set the timeout at a default of 0 ms.
+
+```js
+const promiseDebounce = (fn, ms = 0) =>
+  {
+    let timeoutId;
+    const pending = [];
+
+    return (...args) => new Promise((res, rej) => {
+      clearTimeout(timeoutId);
+      timeoutId = setTimeout(() => {
+        const currentPending = [...pending];
+        pending.lenght = 0;
+        Promise.resolve(fn.apply(this, args)).then(
+          data => {
+            currentPending.forEach(({ resolve }) => resolve(data));
+          },
+          error => {
+            currentPending.forEach(({ reject }) => reject(error));
+          },
+        )
+      }, ms);
+      pending.push({ resolve: res, reject: rej });
+    });
+  }
+```
+
+```js
+//some function returning promise
+const fn = arg => new Promise(resolve => {
+    setTimeout(resolve, 1000, ['resolved', arg]);
+});
+const debounced = promiseDebounce(fn, 200);
+debounced('foo').then(console.log);
+debounced('bar').then(console.log);
+// will log ['resolved', 'bar'] both times.
+```