Simple quiz with javascript

Question

I am trying to make a simple quiz system only using javascript and HTML with no external libraries. But I ran into some problems. The script is giving the wrong solution. Even when I select the right checkboxes it only outputs 1 correct answer. I don't know what exactly am I doing wrong, or if there's an alternative way for doing this.

<div class="quizsection">
    <button onclick="startQuiz()" id="startQuiz">Start Quiz</button>
    <div id="questions"></div> 
</div>

<script>
    //Create Array with questions and solutions
    var allQuestions = [{
        question: "Before Mt. Everest was discovered, whaich mountain was considered to be the highest mountain in the world?",
        choices: ["Mt. Kilimanjaro", "Kanchenjunga", "Mount Everest"],
        correctAnswer: 1
      },

      {
        question: "Does England have a 4th of July?",
        choices: ["Yes", "No", "I don't know"],
        correctAnswer: 0
      },

      {
        question: "What is Rupert the bear's middle name?",
        choices: ["Bear", "He doesn't have one!", "The", "Rupert"],
        correctAnswer: 2
      },

      {
        question: " What can you never eat for breakfast? ",
        choices: ["Dinner", "Something sugary", "Lunch", "Supper"],
        correctAnswer: 0
      },

      {
        question: "If there are three apples and you took two away, how many do you have?",
        choices: ["One", "Two", "None"],
        correctAnswer: 1
      },

      {
        question: "Spell 'Silk' out loud, 3 times in a row. What do cows drink?",
        choices: ["Milk", "Water", "Juice", "Cows can't drink"],
        correctAnswer: 1
      },

      {
        question: "Which is heavier, 100 pounds of rocks or 100 pounds of gold? ",
        choices: ["100 pounds of rocks", "100 pounds of rocks", "They weigh the same"],
        correctAnswer: 2
      },

      {
        question: "Can you spell 80 in two letters?",
        choices: ["AI-TY", "It's not possible", "EIGH-TY", "A-T"],
        correctAnswer: 3
      },

      {
        question: "What question must always be answered ''Yes''?",
        choices: ["What does Y-E-S spell?", "Will everyone die someday?", "Does everyone have a biological mother?", "Are you a human?"],
        correctAnswer: 0
      },

      {
        question: "How many sides does a circle have?",
        choices: ["The back", "None. It's a circle", "Two", "Four"],
        correctAnswer: 2
      },

      {
        question: "What has a tail but no body?",
        choices: ["A human", "A coin", "A cloud"],
        correctAnswer: 1
      },

      {
        question: "What word in the English language is always spelled incorrectly?",
        choices: ["It's possible to spell anything right as long as you learn it", "Shakespeare", "Onomatopoeia", "Incorrectly"],
        correctAnswer: 3
      },

      {
        question: "When do you stop at green and go at red?",
        choices: ["Watermelon!", "Traffic light!", "Garden"],
        correctAnswer: 0
      },

      {
        question: "What rotates but still remains in the same place?",
        choices: ["Bottle (spin the bottle game)", "Clock", "Stairs"],
        correctAnswer: 2
      },

      {
        question: "How can you lift an elephant with one hand?",
        choices: ["Truck", "Use both hands!", "Use a lever", "There is no such thing"],
        correctAnswer: 3
      }
    ];

//Function to start the quiz
function startQuiz(){
    
    var i;
    var j;
    var k;
    for(i=0; i<allQuestions.length; i++){
        document.getElementById("questions").innerHTML +='<form id="question">Q'+(i+1)+': '+ allQuestions[i].question;
        
        for(j=0; j<allQuestions[i].choices.length; j++){
        	document.forms[i].innerHTML += '</div><div class="answer"><input name="q1" value="'+ allQuestions[i].choices[j] +'" id="value4" type="checkbox" />' + allQuestions[i].choices[j] + '<br/>';
         }
     document.getElementById("questions").innerHTML +='</form><br/><br/>';
    }
    
    document.getElementById("questions").innerHTML += '<button onclick="solveQuiz()">Solve Quiz</button>';
    
}

function solveQuiz(){
    	var x;
        var txt = ' ';
        var i = 0;
        var correct = 0; 
        for(i = 0; i < document.forms[i].length;i++) { 
        	x = document.forms[i]; 
        	if(x[i].checked) { 
            	correctAnswer = allQuestions[i].correctAnswer;
                if(x[i].value == allQuestions[i].choices[correctAnswer]){
                	correct += 1;
                }
            }
         }
        document.getElementById("questions").innerHTML += 'Correct answers: ' + correct;
}

</script>

Answer 1

function solveQuiz(){
  var x;
  var txt = ' ';
  var i = 0;
  var correct = 0; 
  for(i = 0; i < document.forms.length;i++) { 
    x = document.forms[i]; 
    for(j = 0; j<x.length; j++){
      if(x[j].checked) { 
        correctAnswer = allQuestions[i].correctAnswer;
        if(x[j].value == allQuestions[i].choices[correctAnswer]){
          correct += 1;
        }
      }
   }
 }
 document.getElementById("questions").innerHTML += 'Correct answers: '+ correct;
} 

you can replace your solveQuiz fn with above block; Its better to use radio instead of checkbox;

    document.forms[i].innerHTML += '</div><div class="answer"><input name="q1" value="'+ allQuestions[i].choices[j] +'" id="value4" type="radio" />' + allQuestions[i].choices[j] + '<br/>';

Answer 2

In your function startQuiz you generate more form with same id and also input inside has always the same id for all questions. Try to concatenate id of these elements with for counter index.