Creating images sliders in JS

Question

I'm trying to create a Previous and a Next button to change the image, the Next button works, but not the Previous button. The first code shown has something wrong that creates a problem which JS says is "Uncaught ReferenceError: prev is not defined" the second one is correct but I can't find a difference between these codes can anyone help, please?

The HTML is correct the problem is probably somewhere in the JS

Code 1 (incorrect):

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 perv() {
var slider = document.getElementById("slider");
num--;
if(num < 0) {
num = images.length-1;
}
slider.src = images[num];
}

Code 2 (correct):

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];
} */

Here is the link to the program the incorrect code is a comment

Answer 1

typo in function name (perv instead of prev) in version one

Answer 2

Here is a good working slider

https://code.sololearn.com/WIBnil7Zreq2/?ref=app

Answer 3

Thank you, Gordon, for pointing that out!