在你的计时器函数中添加一个检查来查看计时器是否已经在运行。如果是，在再次启动计时器之前，清除间隔并将时间重置为它的起始值

let startingCount = 5
let time = startingCount * 60
let spaceForTime = document.getElementById('countdown')
let myTimer = null

function timer() {
  if (myTimer !== null) {
    clearInterval(myTimer)
    time = startingCount * 60
  }

  myTimer = setInterval(function() {
    let seconds = time % 60
    let minutes = Math.floor(time / 60)
    seconds = seconds < 10 ? '0' + seconds : seconds

    if (time >= 0) {
      spaceForTime.innerHTML = `${minutes}:${seconds}`
      time--;

    } else {
      spaceForTime.innerText = "Move onto the next question!"
      clearInterval(myTimer)
    }
  }, 1000)
}

你必须在代码中修改它：

let startingCount = 5
let time = startingCount * 60
let spaceForTime = document.getElementById('countdown')

let myTimer = null

function timer() {
 startingCount = 5;<---------
 if (myTimer !== null) {
   clearInterval(myTimer)
 }

myTimer = setInterval(function timer() {
let seconds = time % 60
let minutes = Math.floor(time / 60)
seconds = seconds < 10 ? '0' + seconds : seconds
time--;
if (time >= 0) {
  spaceForTime.innerHTML = `${minutes}:${seconds}`

} else{ <---------
  spaceForTime.innerText = "Move onto the next question!"
  clearInterval(time)
}
}, 1000);
}

您需要在清除timer()中的前一个间隔后重新声明startingCount和time变量。

let startingCount = 5
let time = 0 // set this inside timer()
let spaceForTime = document.getElementById('countdown')

let myTimer = null

function timer() {

  if (myTimer !== null) {
    clearInterval(myTimer)
  }
  
  time = startingCount * 60 // initialize the time here

  myTimer = setInterval(function timer() {
    let seconds = time % 60
    let minutes = Math.floor(time / 60)
    seconds = seconds < 10 ? '0' + seconds : seconds
    time--;
    if (time >= 0) {
      spaceForTime.innerText = `${minutes}:${seconds}`
    } else {
      spaceForTime.innerText = "Move onto the next question!"
      clearInterval(time)
    }
  }, 1000);
}

<div id="button">
  <button onclick="timer()" id="letsChat">Let's Chat!</button>
</div>
<span id="countdown"></span>

计时器在0：01停止的原因是，首先设置minutes和seconds变量，然后减少time。在设置minutes和seconds变量之前，需要减少时间。

需要考虑的几点：
首先，计时器没有重置，因为您没有重置time的值（正如其他人提到的那样）。
第二，它在0：01停止的原因是因为time--在if条件之前，所以条件在迭代中计算time == -1，它应该显示0秒。执行是：

--> currently showing 0:01.
--> new seconds calculated to be 00
--> time-- results in time = -1
--> condition fails since time < 0
--> does not show updated seconds

要解决这个问题，请将time--移动到interval函数的末尾（也就是if条件之后）。
第三，你的else if条件永远不会被访问，因为你把>=放在了if条件中。它应该被修改为更大的>。
具有上述更改的代码：

let startingCount = .1;
let time = startingCount * 60;
let spaceForTime = document.getElementById('countdown');

let myTimer = null;

function timer() {

  if (myTimer !== null) {
    clearInterval(myTimer);
    time = startingCount * 60;
  }

  myTimer = setInterval(function timer() {
    let seconds = time % 60;
    let minutes = Math.floor(time / 60);
    seconds = seconds < 10 ? '0' + seconds : seconds;
    if (time > 0) {
      spaceForTime.innerHTML = `${minutes}:${seconds}`;

    } else if (time === 0) {
      spaceForTime.innerText = "Move onto the next question!";
      clearInterval(time);
    }
    time--;
  }, 1000);
}

<div id="button">
  <button onclick="timer()" id="letsChat">Let's Chat!</button>
</div>
<span id="countdown"></span>

注：缩短时间为6秒，演示更快