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JavaScript

Posted by luisgomez7
luisgomez7
5,557 Points

I need help understanding the function from https://teamtreehouse.com/library/improve-the-quiz-one-solution.

Please explain to me how the function in this program works. I understand it's using arrays as parameters but I DON'T understand how it knows which arrays to use given that they're not referenced anywhere in the function and why a for loop is required.

const correct = [];
const incorrect = [];
let correctAnswers = 0;

for( let i = 0; i < quiz.length; i++ ) {
 let question = quiz[i][0];
 let answer = quiz[i][1];
 let response = prompt(question);

 if( response.toLowerCase() === answer ) {
    correctAnswers++;
    correct.push(question);
   } else {
  incorrect.push(question);
  }
}

function createListItems(arr) {
  let items = '';
  for (let i = 0; i < arr.length; i++) {
   items += `<li>${arr[i]}</li>`;
  }
  return items;
}

let html = `
   <h1>You got ${correctAnswers} question(s) correct</h1>
   <h2>You got these questions right:</h2>
   <ol>${ createListItems(correct) }</ol>

   <h2>You got these questions wrong:</h2>
   <ol>${ createListItems(incorrect) }</ol>
`;

document.querySelector('main').innerHTML = html;

3 Answers

Steven Parker
Steven Parker
231,261 Points
Steven Parker
Steven Parker
231,261 Points

The array passed as the argument is referenced in the function twice, both to control the loop (arr.length) and within the loop to display each item (${arr[i]}).

The purpose of the loop is to step through the array and cause each item to be displayed in a separate list item.

Steven Parker
Steven Parker
231,261 Points
Steven Parker
Steven Parker
231,261 Points

In any function, parameters act only as placeholders for the arguments that will be provided when the function is called. The names of the parameters are not important other than they must be used consistently within the function.

The function doesn't "know" that the argument will be an array, it is written to require one. The later code could pass something other than an array, but if it did there would be a run-time error.

Each time the function is called, it processes only one array. It has no idea how many arrays there are or how many times it will be called.

luisgomez7
luisgomez7
5,557 Points
luisgomez7
luisgomez7
5,557 Points

Ok but what I don't understand is how the function knows that "arr" references the arrays. I've tried substituting all kinds of parameter names and everything works. Is it because the length property is associated with "arr" in the for loop? If so, how does the function know there's more than one array?

luisgomez7
luisgomez7
5,557 Points
luisgomez7
luisgomez7
5,557 Points

Ok, I think I understand now. Somehow I didn't notice that the code below the function called the function twice, each time referring to a different array in the preceding lines of code. Once again, thank you for your help.