JavaScript NaN with HTML -

i'm trying create small maths game code below, keep getting not-a-number error everytime prompt comes up. i'm not entirely sure problem is. have tried round number doesn't situation. point me in right direction?

function start() {      // define questions    var question = [];     // define answers    var answer = [];    // store user input    var input = [];      var score = 0;      // when user enters answer stored input every prompt    for(var = 0; < 10; i++) {      var randnum = math.floor(math.random() * 100);      var operator = [ '+', '-', '*', '/' ];      var ops = [math.floor(math.random() * operator.length)];        question[i] = randnum + ops[operator] + randnum;        answer[i] = randnum + ops[operator] + randnum;        input[i] = prompt(question[i]);    }      // check answer against stored answer, display 1 of either if statement      for(var = 0; < answer.length; i++) {      if (answer[i] == input[i]) {        var outputresult = document.getelementbyid("write");        outputresult.innerhtml += "<p class = corr>question " + (i+1) + " correct.</p>";      } else {        outputresult.innerhtml += "<p class = incorr>question " + (i+1) + " incorrect. answer should have been " + answer[i] + ".</p>";      }    }  } 

<header>  	<h1>maths game!</h1>  </header>    <button onclick="start()">start</button> <!-- start game -->    <div id="write">  </div>    <div class="score"></div>    </div>

you reference operator array incorrectly. try this:

var operator = ['+', '-', '*', '/']; var ops = math.floor(math.random() * operator.length);  question[i] = randnum + operator[ops] + randnum; answer[i] = eval(question[i]); input[i] = prompt(question[i]);