为什么组件渲染 3 次？

Question

I have a component like this:

import { useEffect, useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);
  console.log("comp run");

  const tick = () => {
    setCount(count + 1);
    console.log(count);
  };

  useEffect(() => {
    const timer = setInterval(tick, 10000);
    console.log("effect run");

    return () => {
      clearInterval(timer);
      console.log("clear func run");
    };
  }, []);

  return <div>{count}</div>;
}

export default Counter;

When the code runs, the console outputs as follows:

Output immediately when the program runs:

---

comp run

effect run

---

after 10 seconds :

---

comp run

0

---

after 10 seconds :

---

comp run

0

---

after 10 seconds :

---

0 (then it keeps increasing by 0 every ten seconds)

---

What I don't understand here is exactly this: "comp run" is printed on the screen 3 times. Why 3 ?

Answer 1

This is because useEffect memoize all values inside it. You can use two ways:

1. Add count to useEffect's dependencies array. And when count changes, useEffect will refreshed.

    useEffect(() => { //Your old code here }, [count]); //Here

2. Create a function inside of useCallback hook and memoize function for better performance. Works like in first way, but dependent by tick fucntion, wich dependent by count state.

    const tick = useCallback(() => { setCount(count + 1); console.log(count); }, [count]); useEffect(() => { const timer = setInterval(tick, 1000); console.log("effect run"); return () => { clearInterval(timer); console.log("clear func run"); }; }, [tick]);