Customize the Quick Launch as Expand/Collapse in SharePoint 2013/2016 Using Code

var quickLaunchLinks = $("div[id$='QuickLaunchMenu'] > ul > li:has('ul') > a");

quickLaunchLinks.prepend("<span class='ms-commentexpand-iconouter ql-icon'>

<img alt='expand' src='/Comm/Comms2/_themes/1/spcommon-B35BB0A9.themedpng?ctag=2' class='ms-commentexpand-icon'></span>");

quickLaunchLinks.closest("li").find("> ul").hide();

quickLaunchLinks.click(function(e) {

    e.preventDefault();

    var childUl = $(this).closest("li").find("> ul");

    var visibleChildUL = childUl.is(":visible")

    if (visibleChildUL) {

        $(this).find(".ql-icon").replaceWith("<span class='ms-commentexpand-iconouter ql-icon'>

<img alt='Expand' src='/Comm/Comms2/_themes/1/spcommon-B35BB0A9.themedpng?ctag=2' class='ms-commentexpand-icon'></span>");

        childUl.slideUp();

    } else {

        $(this).find(".ql-icon").replaceWith("<span class='ms-commentcollapse-iconouter ql-icon'>

<img alt='Collapse' src='/Comm/Comms2/_themes/1/spcommon-B35BB0A9.themedpng?ctag=2' class='ms-commentcollapse-icon'></span>");

        childUl.slideDown();

    }

});