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Bodie Wood
Bodie Wood
Full Stack JavaScript Techdegree Student 4,024 Points

Simple solution.

const quiz = [['What is a Sheepadoodle?','Is a tomato a fruit or veggie?', 'Are birds real?'],
              ['dog', 'fruit', 'yes']];

let numCorrect = 0;

for(i = 0; i <= quiz.length; i++){
  const answer = prompt(`${quiz[0][i]}`).toLowerCase();
  if(answer === quiz[1][i]){

document.querySelector('main').innerHTML = `<h1>You answered ${numCorrect} questions correct.`;

Good work.

In the for statement, you are using i = 0 and what that does, with no let, const or even the lesser used var, is set i = 0 against the global object.

In the browser, that is window in the browser, and global in nodejs.

This would mean that if something else in the global namespace had set the value for i already, your loop has just overwritten it. You can scope the variable to just the for loop by changing it to let i = 0.