Callback

콜백이 무엇인지에 대해서 몰랐다.

유트브를 보았다.

"use strict";

JavaScript is synchornous.
Execude the code block by orfer after hoisting.
hoisting: var, function declaration

console.log("1");
setTimeout(function () {
  console.log("2");
}, 1000);

console.log("3");

"use strict";

// JavaScript is synchornous.
// Execude the code block by orfer after hoisting.
// hoisting: var, function declaration

console.log("1");
setTimeout(() => console.log("2"), 1000);

console.log("3");

// Synchronous callback

function printImmediately(print) {
  print();
}

printImmediately(() => console.log("hello"));

// Asynchronous callback

function printWithDelay(print, timeout) {
  setTimeout(print, timeout);
}

printWithDelay(() => console.log("async callback"), 2000);

// Callback Hell example

class UserStorage {
  loginUser(id, password, onSuccess, onError) {
    setTimeout(() => {
      if (
        (id === "ellie" && password === "dream") ||
        (id === "color" && password === "academy")
      ) {
        onSuccess(id);
      } else {
        onError(new Error("not found"));
      }
    }, 2000);
  }

  getRoles(user, onSuccess, onError) {
    setTimeout(() => {
      if (user === "ellie") {
        onSuccess({ name: "ellie", role: "admin" });
      } else {
        onError(new Error("no access"));
      }
    }, 1000);
  }
}

const userStorage = new UserStorage();
const id = prompt("enter your id");
const password = prompt("enter your password");
userStorage.loginUser(
  id,
  password,
  (user) => {
    userStorage.getRoles(
      user,
      (userWithRole) => {
        alert(
          `Hello ${userWithRole.name}, you have a ${userWithRole.role} role`
        );
      },
      (error) => {
        console.log(error);
      }
    );
  },
  (error) => {
    console.log(error);
  }
);

Promise

https://www.youtube.com/watch?v=JB_yU6Oe2eE&list=PLv2d7VI9OotTVOL4QmPfvJWPJvkmv6h-2&index=13

"use strict";

//Promise is a JavaScript object for asynchronous operation.
//state: pending -> fulfilled or rejected

//Producer vs Consumer

//1. Producer
//when new Promise is created, teh executor runs automatically.

const promise = new Promise((resolve, reject) => {
  // doing some heavy work(network, read files_
  console.log("doing something...");
  setTimeout(() => {
    resolve("ellie");
    // reject(new Error("no network"));
  }, 2000);
});

2. Consumers: then, catch, finally

promise
  .then((value) => {
    console.log(value);
  })
  .catch((error) => {
    console.log(error);
  })
  .finally(() => console.log("finally"));

//3. Promise chaining

const fetchNumber = new Promise((resolve, reject) => {
  setTimeout(() => resolve(1), 1000);
});

fetchNumber
  .then((num) => num * 2)
  .then((num) => num * 3)
  .then((num) => {
    return new Promise((resolve, reject) => {
      setTimeout(() => resolve(num - 1), 1000);
    });
  })
  .then((num) => console.log(num));

//4. Error Handling

const getHen = () =>
  new Promise((resolve, reject) => {
    setTimeout(() => resolve("🐔", 1000));
  });

const getEgg = (hen) =>
  new Promise((resolve, reject) => {
    setTimeout(() => resolve(`${hen} => 🥚`), 1000);
  });

const cook = (egg) =>
  new Promise((resolve, reject) => {
    setTimeout(() => resolve(`${egg} => 🍳`, 1000));
  });

getHen()
  .then((hen) => getEgg(hen))
  .then((egg) => cook(egg))
  .then((meal) => console.log(meal));

reject 부분을 살펴보면

const getEgg = (hen) =>
  new Promise((resolve, reject) => {
    setTimeout(() => reject(new Error(`error! ${hen} => 🥚`)), 1000);
  });

getHen() //
  .then(getEgg)
  .catch((error) => {
    return "🍞";
  })
  .then(cook)
  .then(console.log)
  .catch(console.log);

중간에 에러가 있을시 빵을 받아온다라는 것을 넣어줄수도 있다.

callback-to-promise

콜백 지옥인 코드를 프로미스로 만들 수 있다.

class UserStorage {
  loginUser(id, password) {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        if (
          (id === "ellie" && password === "dream") ||
          (id === "color" && password === "academy")
        ) {
          resolve(id);
        } else {
          reject(new Error("not found"));
        }
      }, 2000);
    });
  }

  getRoles(user) {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        if (user === "ellie") {
          resolve({ name: "ellie", role: "admin" });
        } else {
          reject(new Error("no access"));
        }
      }, 1000);
    });
  }
}

const userStorage = new UserStorage();
const id = prompt("enter your id");
const password = prompt("enter your password");

userStorage
  .loginUser(id, password)
  .then(userStorage.getRoles)
  .then((user) => alert(`Hello ${user.name}, you have a ${user.role} role`))
  .catch(console.log);

async와 await

//async & await
//clear style of using promise :)

//1. async

function fetchUser() {
  return new Promise((resolve, reject) => {
    // do network request in 10 secs....
    resolve("ellie");
  });
}

const user = fetchUser();
user.then(console.log);
console.log(user);

---

async function fetchUser() {
  // do network request in 10 secs....
  return "ellie";
}

---

간단하게 바뀐다.

// 2. await

function delay(ms) {
  return new Promise((resolve) => setTimeout(resolve, ms));
}

async function getApple() {
  await delay(3000);
  return "🍎";
}

async function getBanana() {
  await delay(3000);
  return "🍌";
}

function pickFruits() {
  return getApple().then((apple) => {
    return getBanana().then((banana) => `${apple} + ${banana}`);
  });
}

---

async function pickFruits() {
  const apple = await getApple();
  const banana = await getBanana();
  return `${apple} + ${banana}`;
}


pickFruits().then(console.log);

---

--> 비동기적인 요소들을 await로 관리해준다.
하지만 위에것들은 3초 3초 6초를 기다려야하니까

-> 병렬적으로 만들어준다.
-> Promise

---

async function pickFruits() {
  const applePromise = getApple();
  const bananaPromise = getBanana();
  const apple = await applePromise;
  const banana = await bananaPromise;
  return `${apple} + ${banana}`;
}

---

//3. useful Promise APIs

function pickAllFruits() {
  return Promise.all([getApple(), getBanana()]).then((fruits) =>
    fruits.join(" + ")
  );
}

pickAllFruits().then(console.log);

function pickOnlyOne() {
  return Promise.race([getApple(), getBanana()]);
}

pickOnlyOne().then(console.log);