User:Username/BlockAbuser.js

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mw.loader.using( [ 'mediawiki.util' ] ).then( function () {

    // Only run on Special:AbuseFilter
    if ( mw.config.get( 'wgCanonicalSpecialPageName' ) !== 'AbuseFilter' ) {
        return;
    }

    const seenUsers = new Set();

    // Very strict IPv4 / IPv6 detection
    const ipRegex = /^(?:\d{1,3}\.){3}\d{1,3}$|:/;

    // Find all links inside the abuse log table/content
    $( '#mw-content-text a' ).each( function () {
        const $link = $( this );
        const href = $link.attr( 'href' );
        const text = $link.text();

        if ( !href || !text ) {
            return;
        }

        // Only User: links
        if ( !href.includes( '/wiki/User:' ) ) {
            return;
        }

        const username = decodeURIComponent(
            href.split( '/wiki/User:' )[1]
        ).replace( /_/g, ' ' );

        // Skip IPs
        if ( ipRegex.test( username ) ) {
            return;
        }

        // Only one checkbox per user
        if ( seenUsers.has( username ) ) {
            return;
        }
        seenUsers.add( username );

        // Create checkbox
        const $checkbox = $( '<input>' )
            .attr( {
                type: 'checkbox',
                'data-username': username
            } )
            .css( {
                marginRight: '4px',
                verticalAlign: 'middle'
            } );

        // Insert checkbox before the username link
        $link.before( $checkbox );
    } );

} );