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JavaScript JavaScript Loops, Arrays and Objects Tracking Multiple Items with Arrays Build a Quiz Challenge, Part 1 Solution

Ekaterina Svobodina
Ekaterina Svobodina
4,875 Points
Posted by Ekaterina Svobodina
Ekaterina Svobodina
4,875 Points

JavaScript adds undifined when printing to the page.

So I haven't looked at the solution yet, because I want to understand why this code does what it does. It works, and print to the page what I want except it adds undefind:

This is what I want it to print:

You got 2 question(s) right. You got these question(s) correct:

  1. How many states are there in the USA?

  2. How many oceans are there?

You got these question(s) wrong:

  1. How many continents are there?

And this is what it prints:

You got 2 question(s) right. You got these question(s) correct: undefined

  1. How many states are there in the USA?

  2. How many oceans are there?

You got these question(s) wrong:

undefined

  1. How many continents are there?

Here is my code:

var qAndA = [
  ['How many states are there in the USA?', 51],
  ['How many continents are there?', 7 ],
  ['How many oceans are there?', 5]
];
//var q1 = parseInt(prompt(qAndA[0][0]));
//var q2 = parseInt(prompt(qAndA[1][0]));
//var q3 = parseInt(prompt(qAndA[2][0]));
var i;
var q;
var correct;
var notCorrect;
var correctN = 0;
var notCorrectN = 0;
var messageA;
var messageB;


for ( i = 0; i < qAndA.length; i += 1){
 q = parseInt(prompt(qAndA[i][0]));
if (q === qAndA[i][1]) {
  correctN +=1;
  correct += '<p>' + correctN + '. ' + qAndA[i][0] + '</p>';
} else {
  notCorrectN +=1;
  notCorrect += '<p>' + notCorrectN + '. ' + qAndA[i][0] + '</p>';

}
}

function print(messageA) {
  messageA = document.write('<div><h2>You got ' + correctN + ' question\(s\) right.</h2>' +
   '<b> You got these question\(s\) correct: </b>' +
    correct + '</div>' );
    if (notCorrectN > 0) {
   messageB = document.write(
      '<p><b> You got these question\(s\) wrong: </b></p>' +
      '<p>' + notCorrect + '</p>'); 
    messageA += messageB;
  }
  return messageA;
}

print(messageA);

2 Answers

akak
akak
29,445 Points
akak
akak
29,445 Points

When you define variable like: 

var i;
var q;
var correct;
var notCorrect;

its initial value is "undefined".

If you overwrite variable after declaration with a new value it's not a problem. But if you're going to concatenate, then you need to declare them initially as an empty string.

Ekaterina Svobodina
Ekaterina Svobodina
4,875 Points
Ekaterina Svobodina
Ekaterina Svobodina
4,875 Points

Thank you very much! It worked just like you said.