How to add text before WordPress menu item href

I’m working on a custom project for a client and having an issue adding in data-scroll before the links in WordPress. I want the output to be <a data-scroll href="#">.

Data-scroll is what I am looking to add in just after the a, here href ="#"

Hope someone can help me out.

A Custom Walker would be the best way to add anything anywhere to your wp_nav_menu

You hav to keep the following in your function.php

            <?php
        class MV_Cleaner_Walker_Nav_Menu extends Walker {
            var $tree_type = array( 'post_type', 'taxonomy', 'custom' );
            var $db_fields = array( 'parent' => 'menu_item_parent', 'id' => 'db_id' );
            function start_lvl(&$output, $depth) {
                $indent = str_repeat("\t", $depth);
                $output .= "\n$indent<ul class=\"sub-menu\">\n";
            }
            function end_lvl(&$output, $depth) {
                $indent = str_repeat("\t", $depth);
                $output .= "$indent</ul>\n";
            }
            function start_el(&$output, $item, $depth, $args) {
                global $wp_query;
                $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
                $class_names = $value="";
                $classes = empty( $item->classes ) ? array() : (array) $item->classes;
                $classes = in_array( 'current-menu-item', $classes ) ? array( 'current-menu-item' ) : array();
                $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
                $class_names = strlen( trim( $class_names ) ) > 0 ? ' class="' . esc_attr( $class_names ) . '"' : '';
                $id = apply_filters( 'nav_menu_item_id', '', $item, $args );
                $id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';
                $output .= $indent . '<li' . $id . $value . $class_names .'>';
                $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
                $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
                $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
                $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
                $item_output = $args->before;
                $item_output .= '<a data-scroll'. $attributes .'>';
                $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
                $item_output .= '</a>';
                $item_output .= $args->after;
                $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
            }
            function end_el(&$output, $item, $depth) {
                $output .= "</li>\n";
            }
        }
        ?>

and then you can call the menu by

<?php wp_nav_menu( array( 'menu'=>'Footer Menu 1',
menu_class => 'menu vertical footer-menu',
'walker' => new MV_Cleaner_Walker_Nav_Menu() ) ); ?>