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// 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. createCardAndPoint -> createCard, createPoint

// function is object in JS

function printHello() {

console.log('Hello');

}

printHello();

function log(message) {

console.log(message);

}

log('Hello@');

log(1234);

// 2. 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);

// 3. Default parameters (added in ES6)

function showMessage(message, from = 'unknown') {

console.log(`${message} by ${from}`);

}

showMessage('Hi!');

// 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)}`);

// 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 other functions.

// can be returned by another function

// 1. Function expression

// a function declaration can be called earlier than it is defined. (hoisted)

// 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;

const simpleMultiply = (a, b) => {

// do something more

return a * b;

};

// IIFE: Immediately Invoked Function Expression

(function hello() {

console.log('IIFE');

})();

// Fun quiz time❤️

// function calculate(command, a, b)

// command: add, substract, divide, multiply, remainder

function calculate(command, a, b) {

switch (command) {

case 'add':

return a + b;

case 'substract':

return a - b;

case 'divide':

return a / b;

case 'multiply':

return a * b;

case 'remainder':

return a % b;

default:

throw Error('unknown command');

}

}

console.log(calculate('add', 2, 3));