gistfile1.js

// Store $('body') and $('.mainMenu') in variables outside of the event handler
// This way they only need to be looked up once and not on every click
var $body = $('body');
var $mainMenu = $(".mainMenu");

$body.click(function(e){
    // Since you're only checking the className you can use `hasClass` here instead of `is`
    // http://jsperf.com/jquery-is-vs-hasclass
    if ( $(e.target).hasClass("js-menuToggle") ) {
        $mainMenu.toggleClass("mainMenu--visible");
        $body.toggleClass('js-fixScroll');
    }
    else {
        $mainMenu.removeClass("mainMenu--visible"); 
        $body.removeClass('js-fixScroll');
    }
});