Default Parameters

function greet(greeting, name="Anonymous"){
    console.log(greeting +", "+ name);
}

greet(); // undefined, Anonymous
greet('Hello'); // Hello, Anonymous
greet('Hello', 'Dady'); // Hello, Dady

// ES5
function greet(cb){
    cb();
}

greet(function(){
    console.log("completed"); // completed
})

// ES6
function greet(() => console.log("completed")){
    cb();
}
greet();