JS 기초 강의(ES5+) - # Function (4)

1. 함수 선언 Function declaration

• function name(param1, param2) { body... return; }

• one function === one thing

• naming: doSomething, command, verb / e.g. createCardAndPoint -> createCard, createPoint

• function is object in JS

  function printHello() {
    console.log('Hello'); //
  }
  printHello();
  
  function log(message) {
    console.log(message);
  }
  log('Hello@'); // -> [Hello@]
  log(1234); // -> [1234]

2. 파라미터 Parameters

• primitive parameters: passed by value
• object parameters: passed by reference

  function changeName(obj) {
    obj.name = 'coder';
  }
  const ellie = { name: 'ellie' };
  changeName(ellie);
  console.log(ellie); // -> [{name: "coder"}]

3. Default parameters (added in ES6)

• function showMessage(message, from = 'unknown') {
    console.log(`${message} by ${from}`);
  }
  showMessage('Hi!'); // -> [Hi! by unknown]

4. Rest parameters (added in ES6)

• function printAll(...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));
  }
  printAll('dream', 'coding', 'ellie');

5. Local scope

• let globalMessage = 'global'; // global variable
  function printMessage() {
    let message = 'hello';
    console.log(message); // local variable
    console.log(globalMessage);
    function printAnother() {
      console.log(message);
      let childMessage = 'hello';
    }
    console.log(childMessage); // -> [error]
  }
  printMessage();

6. Return a value

• function sum(a, b) {
    return a + b;
  }
  const result = sum(1, 2); // 3
  console.log(`sum: ${sum(1, 2)}`); // -> [sum: 3]

7. Early return, early exit

• 나쁜 예

  function upgradeUser(user) {
    if (user.point > 10) {
      // long upgrade logic...
    }
  }

• 좋은 예

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