Why My Variable "intervalId" Is Not Getting The Value Updated When I Call The Function "StopTimer" Function?

Here is my simple code: import React, { useState } from 'react'; import './App.css'; function App() { const [time, setTime] = useState(0); var intervalId; function startTim

Solution 1:

Because intervalId is not the same variable that was in scope when startTimer was invoked. It will be undefined in all the subsequent renders. ( caused when time change state )

In React, if you want to "keep a value" across the life cycle of a component, you can use a ref for it:

  // ....
  var intervalId = React.useRef();
  // ....
  intervalId.current = setInterval(() => {
  // ...
  clearInterval(intervalId.current);

Solution 2:

Since the App is invoked everytime you change the state (setTime), you get a new, fresh value of the local variable.

One of options is to include the variable in your state.

function App() {
  var [time, setTime] = useState(0);
  var [intervalId, setIntervalId] = useState(0);

  function startTimer() {
    setIntervalId(intervalId => setInterval(() => {
      setTime(time => time + 1);
    }, 1000));
  }

  function stopTimer() {
    clearInterval(intervalId);
  }

  return (
    <div className="App">
      <div>{time}</div>
      <div>
        <button onClick={startTimer}>Start time</button>
        <button onClick={stopTimer}>Stop time</button>
      </div>
    </div>
  );
}