将clearInterval应用于间隔后，为什么在调用该间隔时该间隔不再起作用？

Question

My game is quite simply you are supposed to click on the start button and move your mouse along the track until you reach the end. At the end the timer stops and shows your score. My problem lies with the timer. The timer work perfectly fine the first time but when you click reset to play again it only calls the interval once. Why?

Link to my full code: http://www.codecademy.com/TictacTactic/codebits/AQBK4L/edit

Thank you in advance!

JAVA SCRIPT

var clicked = false;
var score = 1000;
var timer = setInterval(function(){countDown()}, 250);

$(document).ready(function() {
$('#start').click(function() {
if(!clicked){
    clicked = true;
    countDown();
}
});

$("#lava").mouseover(function(){
if(clicked){
    stopTimer();
    score = 0;
    $("#points").html(score);
}
});

$("#end").mouseover(function(){
if(clicked){
    stopTimer();
    $("#points").html(score);
    clicked = false;
}
});

$('#reset').click(function() {
if(clicked || !clicked){
    clicked = false;
    score = 1000;
    $('#points').html(score);
}
});

});

function countDown() {
score = score - 1;
}

function stopTimer() {
clearInterval(timer);
}

Answer 1

your code var timer = setInterval(function(){countDown()}, 250); is at the top and will be executed before the methods are even loaded. Add at the end of the ready() , look at the bottom:

var clicked = false;
var score = 1000;
var timer = null;

$(document).ready(function() {
  $('#start').click(function() {

  if(!clicked){
    clicked = true;
    score = 1000;
    timer = setInterval(function(){countDown()}, 250);
    //countDown();
   }
  });

 $("#lava").mouseover(function(){
   if(clicked){
    stopTimer();
    score = 0;
    $("#points").html(score);
   }
 });

 $("#end").mouseover(function(){
   if(clicked){
    stopTimer();
    $("#points").html(score);
    clicked = false;
   }
});

 $('#reset').click(function() {
  if(clicked || !clicked){
    clicked = false;
    score = 1000;
    $('#points').html(score);
  }
});



});

function countDown() {
  score = score - 1;
  $('#points').html(score);
  }

function stopTimer() {
  clearInterval(timer);
  }