React Hook useEffect has a missing dependency warning

This warning means that I have not included all the variables I use in the useEffect hook in the dependency array, and the useEffect hook may not function as intended.

Let's take a look at the different variations of the useEffect hook.

useEffect hook without dependency array

The useEffect hook without a dependency array runs every time the component re-renders. That is equivalent to the componentDidUpdate() lifecycle hook in class components.

useEffect(() => {
    console.log("some text") //executes in every component re-render
})

That may not be necessary and may negatively impact performance.

useEffect hook with empty dependency array

I can control the useEffect hook's execution by adding a dependency array. The useEffect hook with an empty dependency array runs only when the component initially mounts. That is equivalent to the componentDidMount() lifecycle hook in class components.

useEffect(() => {
    console.log("some text") //executes only when the component initially mounts
},[])

Now, what if I use a state variable inside the useEffect hook? The below hook will run when the component mounts. After that, any changes to the text variable will not affect this hook. The if(text){ condition may never be executed.

useEffect(() => {
  if(text){
    console.log(text)
  }
}, [])//React Hook useEffect has a missing dependency warning

useEffect hook with dependencies in the dependency array

I can overcome this limitation by including the text variable in the dependency array. In that way, my useEffect will run whenever the text state variable changes its value. Here is the complete code example to demonstrate that.

import { useEffect, useState } from "react";
const App = () => {
  const [text, setText] = useState<string>("")
  useEffect(() => {
      if(text){
        console.log(text)//runs when text changes
      }
  }, [text])
  return (
    <div>
        <textarea onChange={(e) => setText(e.target.value)}></textarea>
    </div>
  );
}
export default App;

The same rules apply when I use props inside my useEffect hook. The hook must know when to re-render when the props change. Otherwise, the useEffect hook ends up with stale variables that can lead to unexpected behavior.

In the below code, the useEffect runs when the component mounts. Then it is unaware of any changes to the props.text.

useEffect(() => {
  if(props.text){
    console.log(props.text)
  }
}, []) //React Hook useEffect has a missing dependency warning

I can solve that potential issue by adding the props.text to the dependency array.

useEffect(() => {
  if(props.text){
    console.log(props.text)
  }
}, [props.text])