Greasy Fork

I updated it to have some keywords and changed the description

// ==UserScript==
// @name        MangaNelo Scripts (Next/Prev Chapter & Image Resize)
// @namespace   ew0345
// @match       *://manganelo.com/chapter/*/*
// @grant       none
// @version     1.1
// @author      ew0345, Qther
// @description Allows L/R arrow key navigation between chapters & reize chapter images using / key.
// ==/UserScript==

var img = ['.container-chapter-reader img'];
var wper = 100;

function resize() {
    var inp = prompt('Image Width / %\n\'autoh\' to fit images to page height,\n\'autow\' to fit images to page width.');
    wper = inp.valueOf().toLowerCase();
    console.log(wper);

    if (wper === "autow" || wper === "w" || wper === "fitw") {
        var pageWidth = $(document).width();
        console.log(pageWidth);
        for (var i = 0; i < document.querySelectorAll(img).length; i++) {
            document.querySelectorAll(img)[i].style.width = pageWidth / document.querySelectorAll(img)[i].width * 100 + '%';
        }
    } else if (wper === "autoh" || wper === "h" || wper === "fith" || wper === "4koma") {
        var pageWidth = $(document).width();
        var pageHeight = $(window).height();
        console.log(pageHeight);
        for (var i = 0; i < document.querySelectorAll(img).length; i++) {
            document.querySelectorAll(img)[i].style.width = pageWidth / document.querySelectorAll(img)[i].width * 100 + '%';
            document.querySelectorAll(img)[i].style.width = pageHeight / document.querySelectorAll(img)[i].height * 100 + '%';
        }
    } else {
        for (var i = 0; i < document.querySelectorAll(img).length; i++) {
            document.querySelectorAll(img)[i].style.width = wper + '%';
        }
    }
}

window.onkeydown = function(e) {
    switch (e.key) {
        case "ArrowLeft":
            $('a:contains("PREV CHAPTER")').length > 0 ? document.location = $('a:contains("PREV CHAPTER")').attr('href') : console.error('Already on last chapter.');
            break;
        case "ArrowRight":
            $('a:contains("NEXT CHAPTER")').length > 0 ? document.location = $('a:contains("NEXT CHAPTER")').attr('href') : console.error('Already on first chapter.');
            break;
        case "/":
            resize();
            break;
        default:
            break;
    }
}