Can not get clientWidth using useEffect

I want to get windowsize of the browser and according to this condition I want to render something in the component.
When I try to get clientWidth using useEffect, I get the value. However, I can not assign it the variable which I defined it in useState.

This is my code. and sometimes I’m getting the following error which I don’t understand because I don’t assign anything to constant variable. I try to assign variable of useState.

import React, { useState, useEffect } from 'react';

export default function App() {
  const [isMobile, setIsMobile] = useState(false);

  useEffect(() => {
    if (document.documentElement.clientWidth <= 400) {
      console.log(document.documentElement.clientWidth);
      setIsMobile = true;
    }
  }, []);

  return (
    <div>
      {(() => {
        console.log(isMobile);
        if (isMobile === true) {
          return <div>This is Mobile</div>;
        } else {
          return <div>This is normal</div>;
        }
      })()}
    </div>
  );
}

And this is the written version in stackblitz.

https://stackblitz.com/edit/react-i9ewp6?file=src/App.js

>Solution :

You are trying to assign a value to the setIsMobile state updater function, which is declared const, so you can’t reassign a value to it anyway. You need to call the function with the value you want to update state to.

useEffect(() => {
  if (document.documentElement.clientWidth <= 400) {
    console.log(document.documentElement.clientWidth);
    setIsMobile(true);
  }
}, []);

or more succinctly

useEffect(() => {
  setIsMobile(document.documentElement.clientWidth <= 400);
}, []);

Since isMobile is a boolean value, it’s considered anti-pattern to compare it against true|false, just use its "boolean-ness" directly.

return (
  <div>
    {isMobile ? (
      <div>This is Mobile</div>
    ) : (
      <div>This is normal</div>
    )}
  </div>
);

Also, if you want to log values do it in an useEffect hook as an intentional side-effect.

useEffect(() => {
  console.log(isMobile);
}, [isMobile]);