I'm trying to dive deeper into rxjs and found an issue where the input field I'm trying to debounce dispatches an event on every keypress, the debounce only holds the output but results in a tree like:

a
as(delay - waits 200ms, then fires the rest synchronously)
asd
asdf
asdfg 
....

The same code works as expected in a class ponent() but cannot understand why it doesn't work with stateless ponents. Here's an example: - you can see the useState update fires for every keystroke.

Thanks a lot.

I'm trying to dive deeper into rxjs and found an issue where the input field I'm trying to debounce dispatches an event on every keypress, the debounce only holds the output but results in a tree like:

a
as(delay - waits 200ms, then fires the rest synchronously)
asd
asdf
asdfg 
....

The same code works as expected in a class ponent(https://stackoverflow./a/44300853/1356046) but cannot understand why it doesn't work with stateless ponents. Here's an example: https://stackblitz./edit/react-hzhrmf - you can see the useState update fires for every keystroke.

Thanks a lot.

• javascript
• rxjs
• debouncing

2 Answers 2

React continuously calls your function to render the ponent. Therefore the Subject is continuously recreated.

Using a factory with useState to keep the subject and working with useEffect to make sure the subscription is only made once should fix your issue.

Something like this :

import React, { Component, useState, useEffect, useRef } from 'react';
import { render } from 'react-dom';
import { debounceTime, map, tap, distinctUntilChanged } from 'rxjs/operators';
import { fromEvent, Subject } from 'rxjs';

import './style.css';
const App = props => {
  const [queryName, setQueryName] = useState("");
  const [debouncedName, setDebouncedName] = useState("");
  const [onSearch$] = useState(()=>new Subject());
  useEffect(() => {
    const subscription = onSearch$.pipe(
      debounceTime(400),
      distinctUntilChanged(),
      tap(a => console.log(a))
    ).subscribe(setDebouncedName);
  }, [])
  const handleSearch = e => {
    setQueryName(e.target.value);
    onSearch$.next(e.target.value);
  };

  return (
    <div>
      <input
        placeholder="Search Tags"
        value={queryName}
        onChange={handleSearch}
      />
      <p>Debounced: {debouncedName}</p>
    </div>
  );
}

render(<App />, document.getElementById('root'));

Here is a version as custom Hook as this might be used multiple times in a form and can otherwise clutter your code.

function useDebounce<T = any>(time: number, defaultValue: T): [T, (v: T) => void] {
  let [value, setValue] = React.useState<T>(defaultValue);
  let [value$] = React.useState(() => new Subject<T>());
  React.useEffect(() => {
    let sub = value$.pipe(debounceTime(time)).subscribe(setValue);
    return () => sub.unsubscribe();
  }, [time, value$]);
 return [value, (v) => value$.next(v)];
}

//useage: 
let [value,setValue] = useDebounce(200,"");