SPAccessDenied.js

function getQueryStringParam(searchKey) {
    // get URI object
    var uri = new URI(location.href);
    
    // get query params
    var queryParams = uri.getQueryAsObject();

    // get queryParam keys
    var queryParamKeys = Object.keys(queryParams);
    
    // get index of key, case insensitive
    var keyIndex = queryParamKeys.map(key => key.toLowerCase()).indexOf(searchKey.toLowerCase());

    // handle key not found
    if(keyIndex === -1)
        return undefined;

    // get actual key from queryParams
    var key = queryParamKeys[keyIndex];
    
    // return value
    return queryParams[key];
}

var source = getQueryStringParam("source");
window.location.href = source;