Why is this an issue?

React’s useState hook setter function should not be called directly in the body of a component, as it would produce an infinite render loop. A re-rendering occurs whenever the state of a component changes. Since a hook setter function changes the component’s state, it also triggers re-rendering.

The loop "state updates → triggers re-render → state updates → triggers re-render → …" will continue indefinitely.

import { useState } from "react";

function ShowLanguage() {
    const [language, setLanguage] = useState("fr-FR");

    setLanguage(navigator.language); // Noncompliant: causes an infinite loop

    return (
      <section>
        <h1>Your language is {language}!</h1>
        <button onClick={() => setLanguage("fr-FR")}>Je préfère le français</button>
      </section>
    );
}

Instead, the setter function should be called from an event handler.

import { useState } from "react";

function ShowLanguage() {
    const [language, setLanguage] = useState(navigator.language);

    return (
      <section>
        <h1>Your language is {language}!</h1>
        <button onClick={() => setLanguage("fr-FR")}>Je préfère le Français</button>
      </section>
    );
}

Why is this an issue?

Resources

Documentation