MediaWiki:Access.js

var controller_item_id = 96;

var wbEStruct = [
				'claims.P3', // location of P3 relations in wbEntity
				'mainsnak.datavalue.value', // location of ID properties in related entity
				'numeric-id' // ID property name which checked 
				]; 

// check if we are on a page of a loaded Entity
if ( typeof mw.config.values.wbEntity !=  "undefined" ) {
	console.log("entity is defined")
	// get wbEntity as json
	var obj = JSON.parse ( mw.config.values.wbEntity );
	console.log("parsed object")
	// check if we have P3 relations and iterate through
	if ( typeof obj.claims.P3 != "undefined" ) {
		console.log("object has P3")
		for( var i = 0; i < dive(obj, wbEStruct[0]).length; i++ ) {
			console.log("dived in")		
			// check if we have controller id constructed from wbEntity 
			if( dive(dive(obj, wbEStruct[0])[i] , wbEStruct[1])[wbEStruct[2]] === controller_item_id ) {
				console.log("it's a controller")		
				var $title2 = $( '.wikibase-title' ),
				qId2 = $title2.find( '.wikibase-title-id' ).text().replace( /[()]/g, '' );

				api.get( {
				    action: 'expandtemplates',
				    text: '{{MailtoAccess|'+qId+'}}'
				} ).done( function ( data ) {
				    mw.notify( $('<a href="'+data.expandtemplates["*"]+'"> Do a SAR to this controller!</a>') , { autoHide: false } );
				} );
		
			}
		}
	}
	// Else let's do nothing;
}

// helper function for diving into a part of array
function indexinterpolate(obj,i) { return  (obj[i] != undefined) ? obj[i] : obj}; 

// dive selects matrix.a.b.c.d from the array called matrix and 'a.b.c.d' as string
// if a.b.c.d does not exists, it returns the substructure until the substructure exists, if d does not exist, it returns matrix.a.b.c e.g.

function dive(array, read){
	return read.split('.').reduce(indexinterpolate, array);
}