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JavaScript Object-Oriented JavaScript (2015) Practice Project User Interface Code

Sirin Srikier
Sirin Srikier
7,671 Points

Uncaught TypeError: Cannot set property 'innerHTML' of null

I'm not sure why I'm getting this error. Please help. https://w.trhou.se/klufprtbgv

var QuizUI = {
  displayNext: function() {
    if (quiz.hasEnded()) {
    } else {
  displayQuestion: function() {
    this.populateIdWithHTML("question", quiz.getCurrentQuestion().text); 
  displayChoices: function() {
    var choices = quiz.getCurrentQuestion().choices;

    for(var i=0; i < choices.length; i++) {
      this.populateIdWithHTML("choice", i, choices[i]);
      this.guessHandler("guess" + i, choices[i]);
  displayScore: function() {
    var gameOverHTML = "<h1>Game Over</h1>";
    gameOverHTML += "<h2> Your score is: " + quiz.score + "</h2>";
    this.populateIdWithHTML("quiz", gameOverHTML);
  populateIdWithHTML: function(id, text) {
    var element = document.getElementById(id);
    element.innerHTML = text;
  guessHandler: function(id, guess) {
    var button = document.getElementById(id);
    button.onclick = function() {
  displayProgress: function() {
    var currentQuestionNumber = quiz.currentQuestionIndex + 1;
    this.populateIdWithHTML("progress", "Question" + currentQuestionNumber + " of " + quiz.questions.length);

1 Answer

Steven Parker
Steven Parker
228,077 Points

The snapshot is very helpful and you don't need to paste in code separately with it.

So here's what's happening:

  • As the program starts, app.js calls "QuizUI.displayNext()" on line 12.
  • Then in quiz_ui.js, eventually line 7 calls "displayChoices()".
  • Then line 18 calls "populateIdWithHTML("choice", i, choices[i])" (when "i" is 0)
  • Then line 28 calls "getElementById" passing the argument "choice"
  • but there is no element in the HTML with the id "choice", so the result is null
  • on line 29, attempting to set a property on null causes the error

Also note that on line 18, "populateIdWithHTML" is passed 3 arguments, but it is defined (on line 27) to take only 2.

Perhaps you meant to concatenate the index to the name on line 18?

      this.populateIdWithHTML("choice" + i, choices[i]);