function.js

"use strict";

// Function
// - fundamental building block in the program
// - subprogram can be used multiple times
// - performs a task or calculates a value

// 1. Function declaration
// function name(param1, param2) { body... return; }
// one function === one thing
// naming: doSomething, command, verb
// e.g. createCarAndPoint -> vreateCared, createPoint
// function is obect in JS
function prinHello() {
  console.log("Hello");
}
prinHello();

function log(message) {
  console.log(message);
}
log("안뇽하셍용");
log(1234);

// 2. Parameter
// premitive parameters: passed by value
// object parameters: passed by reference
function changeName(obj) {
  obj.name = "coder";
}
const och = { name: "och" };
changeName(och);
console.log(och);

// 3. Default parameters (added in ES6)
function showMessage(message, from = "unknown") {
  console.log(`${message} by ${from}`);
}
showMessage("Hi!");

// 4. Rest parameters (added in ES6)(배열형태)
function prinAll(...args) {
  for (let i = 0; i < args.length; i++) {
    console.log(args[i]);
  }
  for (const arg of args) {
    console.log(arg);
  }
  args.forEach((arg) => console.log(arg));
}
prinAll("dream", "coding", "och");

// 5. Local scope
let globalMessage = "global"; // global variable
function printMessage() {
  let message = "hello";
  console.log(message); // local variable
  console.log(globalMessage);
}
printMessage();

// 6. Return a value
function sum(a, b) {
  return a + b;
}
const result = sum(1, 2); // 3
console.log(`sum: ${sum(1, 2)}`);

// 7. Early return, early exit
// bad
function upgradeUser(user) {
  if (user.point > 10) {
    // long upgrade logic ...
  }
}

// good
function upgradeUser(user) {
  if (user.point <= 10) {
    return;
  }
  // long upgrade logic...
}

// First-class function
// functions are treated like any other variable
// can be assigned as a value to variable
// can be passed as an argument to oher functions.
// can be returned by another function

// 1. Function expression
// a function declaration can be called earlier than it is defiend.(hoised)
// a function expression is created when the execution reaches it.
const print = function () {
  // anonymous function
  console.log("print");
};
print();
const printAgain = print;
printAgain();
const sumAgain = sum;
console.log(sumAgain(1, 3));

// 2. Callback function using function expression
function randomQuiz(answer, printYes, printNo) {
  if (answer === "love you") {
    printYes();
  } else {
    printNo();
  }
}
// anonymous function
const printYes = function () {
  console.log("yes!");
};

// named function
// better debugging in debugger's stack traces
// recursions
const printNo = function print() {
  console.log("no!");
};
randomQuiz("wrong", printYes, printNo);
randomQuiz("love you", printYes, printNo);

// Arrow function
// always anonymous
const simplePrint = function () {
  console.log("simplePrint!");
};

const simplePrint = () => console.log("simplePrint!");
const add = (a, b) => a + b;

// IIFE : Immediatley Invoked Function Expression ()로 묶어 바로 실행
(function hello() {
  console.log("IIFE");
})();

// Quiz!