为什么使用setInterval（）会抛出“未定义的函数”错误？

Question

在我的应用程序中，我需要定期调用函数。为实现这一目标，我使用的是setInterval() javascript函数。但是由于某种原因，它给出了未定义函数的错误。

我的JS代码附在下面：

var choice;  //counts how many choices have been made 
choice = 0;
var choice1; //stores index of first card picked
var choice2; //stores index of second card picked
var choice3; //stores index of third card picked
var choice4; //stores index of fourth card picked

var counter = 0;              //counts matches made
var numAttempts = 0;          //counts attempts made
var backcard ="img/nyaa.png"; //shows back of card when turned over
var numOfMatches= 3;

var faces = [];   //array to store card images
faces[0] = 'img/bub.jpg';
faces[1] = 'img/bub.jpg';
faces[2] = 'img/mustache.jpg';
faces[3] = 'img/mustache.jpg';
faces[4] = 'img/bub.jpg';
faces[5] = 'img/bub.jpg';
faces[6] = 'img/hover.jpg';
faces[7] = 'img/hover.jpg';
faces[8] = 'img/hover.jpg';
faces[9] = 'img/hover.jpg';
faces[10] = 'img/mustache.jpg';
faces[11] = 'img/mustache.jpg';


// Responds to click on a table cell indicating card selected
function choose(card) {

  if (choice === 0) {   // if first pick, identify card selected
    choice1 = card;
    document.images[card].src = faces[card];
    choice = 1;    //one pick made
    // console.log(choice1);
  }
  if (choice === 1) {   // second pick, identify card selected
    choice2 = card;
    document.images[card].src = faces[card];
    choice = 2;
    // console.log(choice2);
    return choice2;
  }
  if (choice === 2) {   // third pick, identify card selected
    choice3 = card;
    document.images[card].src = faces[card];
    choice = 3;  
    // console.log(choice3);
    return choice3;
  } else
     choice = 4; { //fourth pick, identify card selected

  choice4 = card;
  document.images[card].src = faces[card];
  choice = 4;
  // console.log(choice4);
  tid = setInterval("checker()",1000); //use timer to pause so user can see selections
   return choice4;
}

// Checks to see if a match is made
function checker() {
  clearInterval(tid); //stops timer
  numAttempts++;      //adds 1 to attempts
  document.getElementById("attempts").innerHTML = numAttempts; //display number of attempts
  if (faces[choice1] === faces[choice2] === faces[choice3] === faces[choice4]) {   //if a match is selected
    counter++;    //adds an increment to matches
    if (counter === numOfMatches) {  //if matches made = maximum possible matches, display message
      alert("You won.\n It took you " + numAttempts + " tries.\n Refresh the page or press restart to play again.");
    }
    choice = 0;
    return ;
  } else {  //if no match made, turn cards back over
    document.images[choice1].src = backcard;
    document.images[choice2].src = backcard;
    document.images[choice3].src = backcard;
    document.images[choice4].src = backcard;
    choice = 0;
    return;
  }      
}

faces = shuffle(faces);

function shuffle(dealer){
  function shuffle() {}
    for(var j, x, i = dealer.length; i; j = Math.floor(Math.random() * i), x = dealer[--i], dealer[i] = dealer[j], dealer[j] = x);
    return dealer;
  };
  function reset(){
    location.reload(); //restarts page and game
  }
}

任何帮助将不胜感激！