Default Function Argument

ES5

function makeAjaxRequest(url, method) {
    if (!method) { 
        method = 'GET'; 
    }
}

ES6 
function makeAjaxRequest(url, method = 'GET') {

}

//More
function generateId() {
    return Math.random() * 999999; 
}

function createUser(id = generateId()){

}