Cannot read property 'style' of undefined in Javascript Function

Question

I have this Javascript function butwhen i run it it says 'Cannot read property 'style' of undefined at showSlides'

 var slideIndex = 1;
showSlides(slideIndex);

// Next/previous controls
function plusSlides(n) {
  showSlides(slideIndex += n);
}

// Thumbnail image controls
function currentSlide(n) {
  showSlides(slideIndex = n);
}

function showSlides(n) {
  var i;
  var slides = document.getElementsByClassName("mySlides");
  var dots = document.getElementsByClassName("dot");
  if (n > slides.length) {slideIndex = 1} 
  if (n < 1) {slideIndex = slides.length}
  for (i = 0; i < slides.length; i++) {
      slides[i].style.display = "none"; 
  }
  for (i = 0; i < dots.length; i++) {
      dots[i].className = dots[i].className.replace(" active", "");
  }
  slides[slideIndex-1].style.display = "block"; 
  dots[slideIndex-1].className += " active";
}

I suppose the error is from slides[slideIndex-1].style.display = "block"; . Try console logging your slideIndex just before this line — Prasanna
– Prasanna, Commented Mar 23, 2018 at 8:01
@Ernest Des-Bordes Do you always have at least one element with class mySlides? Because if you don't then when you are assigning block it will give you an error. — Leo Odishvili
– Leo Odishvili, Commented Mar 23, 2018 at 8:06

Dathan · Accepted Answer · 2018-03-23 08:13:13Z

2

if (n < 1) {slideIndex = slides.length - 1}

slideIndex cannot be slides.length, The last element of slides will be present at slides.length - 1, not slides.length.

answered Mar 23, 2018 at 8:13

Dathan

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