Sharepoint - How to get file or list item icon in SharePoint 2013 using javascript

SharePoint has already lots of icons available in the DOCICON.xml file (14-Hive at C:/Program Files/Common Files/Microsoft Shared/web server extensions/14/TEMPLATE/XML)

Simplest way could be to load based on file extension "/_layouts/images/icXXX.gif (where icXXX, would be icdoc.gif, idpdf.gif, etc.)

You can use the script below :

<script type="text/javascript">
    function getFileTypeIcon(fName, size) {
    var dfd = $.Deferred();
    $.ajax({
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/maptoicon(filename='" + fName + "', progid='', size=" + size + ")",
    method: "GET",
    headers: { "Accept": "application/json; odata=verbose" },
    success: function (data) {
        dfd.resolve(_spPageContextInfo.webAbsoluteUrl + "/_layouts/15/images/" + data.d.MapToIcon);
    },
    error: function (errMsg) {
        if (errMsg.responseText) {
            dfd.reject(errMsg.responseText);
        }
    }
    });    
    return dfd.promise(); 
}
$(document).ready(function() {
    // size = 0 for small and 1 for large
    getFileTypeIcon('https://someurl/someName.pdf', 1).then(function (result) {
        // Write the complete url to file icon
        console.log(result);
    });
});
</script>