6 ways to declare js function

normal function

function sayHello(firstName, lastName) {
  return `hello ${firstName} ${lastName}`;
}

function expression

const sayHello2 = function(firstName, lastName) {
  return `hello ${firstName} ${lastName}`;
};

arrow function

const sayHello3 = (firstName, lastName) => `hello ${firstName} ${lastName}`;

if you want to return an object in arrow function, we could do something like below

const makeAToy = (color, size) => ({ toyColor: color, toySize: size });

Immediately Invoked Function Expression

the function will be executed immediately

(function(firstName, lastName) {
  console.info(`hello ${firstName} ${lastName}`);
})('Amy', 'Li');

methods in object

const greeting = {
  word: 'greeting',
  sayHello1(firstName, lastName) {
    console.log(`${this.word} ${firstName} ${lastName}`);
  },
  sayHello2(firstName, lastName) {
    console.log(`${this.word} ${firstName} ${lastName}`);
  },
  sayHello3: (firstName, lastName) => {
    console.log(`${greeting.word} ${firstName} ${lastName}`);
  },
};

call back function

setTimeout(() => console.log('time to do something'), 1000);