not able to understand the use of the following elements in the given code

Question

what is the use of:- var num=0 in first line of code, num++ in in 4th line, if(num>=images.length) {
num=0 } in 5th and 6th line, slider.src[num] in 7th line, num-- in 10th line,
if(num<0){
num=image.length-1 in 12th line
GIVEN CODE :-
var images = [
"http://www.sololearn.com/uploads/slider/1.jpg",
"http://www.sololearn.com/uploads/slider/2.jpg",
"http://www.sololearn.com/uploads/slider/3.jpg"
];
var num = 0;
function next() {
var slider = document.getElementById("slider");
num++;
if(num >= images.length) {
num = 0;
}
slider.src = images[num];
}
function prev() {
var slider = document.getElementById("slider");
num--;
if(num < 0) {
num = images.length-1;
}
slider.src = images[num];
}
REQUEST:- please upvote this question

Answer 1

var num = 0 initializes a variable, which stores the current index of the image shown in the slider.
num++ increments the variable.
When you call next() and "num" becomes 4, "num = 0" is executed, which is used to select a URL from the array below the line.
slider.src = images[num]
images[num] takes an element with an index equal to "num" from the array "images".
slider.src = images[num] sets the string to slider.src which represents an image shown on the page.
num-- dencrements the variable.
When you call prev() and "num" becomes -1, "num = images length - 1" is executed, which is used to select a URL from the array below the line.
image.length returns the number of elements in the array.
image.length - 1 returns the last index of the array.
As you can see "num" can be any value from 0 to 3 (the last index in the array "images").