JavaScript asynchrone - Éliminez les confusions

Remarque : J'ai expliqué l'ensemble du sujet étape par étape, mais n'hésitez pas à passer à n'importe quelle section où vous avez une question ou avez besoin de précisions.

Avant d'entrer dans le javascript asynchrone, il est important de comprendre ce qu'est le javascript synchrone et pourquoi nous aurons besoin d'une manière asynchrone pour écrire du code javascript, n'est-ce pas ?

Qu’est-ce que le JavaScript synchrone ?

En programmation synchrone, les tâches sont exécutées les unes après les autres, de manière séquentielle. La tâche suivante ne peut démarrer qu'une fois la tâche en cours terminée. Si une tâche prend beaucoup de temps, tout le reste doit attendre.

Pensez-y comme si vous faisiez la queue dans une épicerie : si la personne devant vous a beaucoup d'articles et met beaucoup de temps à payer, vous devez attendre votre tour jusqu'à ce qu'elle ait fini avant de pouvoir continuer.

console.log("Start cooking");

for (let i = 0; i < 5; i++) {
  console.log("Cooking dish " + (i + 1));
}

console.log("Finish cooking");

Voici ce qui se passe :

1. Il imprime "Démarrer la cuisson".

2. Ensuite, il entre dans une boucle et imprime chaque "Plat de cuisson X" l'un après l'autre.

3. Seulement une fois la boucle terminée, il imprime "Terminer la cuisson".

Dans ce cas, le code s'exécute dans l'ordre et rien d'autre ne peut se produire jusqu'à ce que la tâche en cours (cuisson de chaque plat) soit terminée. Imaginez si un plat prenait 10 minutes à cuire : tout le reste devrait attendre que ce plat soit terminé.

Qu’est-ce que le JavaScript asynchrone ?

Dans la programmation asynchrone, les tâches peuvent être démarrées et pendant qu'elles sont encore en cours d'exécution (par exemple, en attente de données d'un serveur), d'autres tâches peuvent continuer à s'exécuter. Vous n’êtes pas obligé d’attendre la fin d’une tâche avant d’en commencer une autre.

Pensez-y comme si vous passiez une commande dans un restaurant : vous commandez votre nourriture, et pendant qu'elle est préparée, vous pouvez continuer à parler à vos amis ou à consulter votre téléphone. Une fois la nourriture prête, le serveur vous l'apportera.

Exemple de code asynchrone :

console.log("Start cooking");

setTimeout(() => {
  console.log("Cooking is done!");
}, 3000); // Simulate a task that takes 3 seconds

console.log("Prepare other things while waiting");

Voici ce qui se passe :

1. Il imprime "Démarrer la cuisson".

2. La fonction setTimeout démarre une minuterie de 3 secondes. Mais au lieu d'attendre, JavaScript passe immédiatement à la ligne suivante.

3. Il imprime "Préparer d'autres choses en attendant".

4. Après 3 secondes, le minuteur se termine et imprime "La cuisson est terminée !".

---

Il existe trois manières principales d'écrire du JavaScript asynchrone, et elles sont :

1. Rappels

2. Promesses

3. Async/Attendre

Voici les principales approches pour gérer le code asynchrone en JavaScript.

Rappels

Une fonction de rappel en JavaScript est une fonction que vous passez en argument à une autre fonction. L'idée de base est que vous transmettez ou définissez une fonction comme argument à une autre fonction, et cette fonction transmise est appelée une « fonction de rappel ». Le rappel est appelé (ou "invoqué") une fois qu'une certaine tâche est terminée, souvent après une tâche asynchrone, comme récupérer des données sur un serveur.

Cela permet à votre fonction principale de continuer à faire autre chose sans attendre la fin de la tâche. Lorsque la tâche est terminée, le rappel est déclenché pour gérer le résultat.

function mainFunc(callback){
  console.log("this is set before setTimeout")
  callback()
  console.log("this is set after setTimeout")
}

function cb(){
  setTimeout(()=>{
    console.log("This is supposed to be painted after 3 second")
  },3000)
}

mainFunc(cb)

Ce code illustre le concept de callback en JavaScript. Voici comment cela fonctionne :

1. mainFunc prend une fonction callback comme argument.

2. À l'intérieur de mainFunc, le callback est exécuté immédiatement, mais comme le callback lui-même (la fonction cb) contient un setTimeout, il planifie l'exécution du console.log après 3 secondes.

3. Pendant ce temps, mainFunc continue de s'exécuter, imprimant des messages avant et après avoir rappelé le rappel.

4. Après 3 secondes, le setTimeout à l'intérieur du rappel se termine et le message retardé est imprimé.

Cela montre comment la fonction principale continue ses tâches sans attendre la fin de l'opération asynchrone (le délai de 3 secondes dans le rappel).

Qu’est-ce qu’un enfer de rappel et quand se produit-il ?

L'enfer des rappels fait référence à une situation en JavaScript dans laquelle plusieurs rappels sont imbriqués les uns dans les autres de manière ingérable, ce qui rend le code difficile à lire, à maintenir et à déboguer. Cela ressemble souvent à une pyramide de fonctions imbriquées, où chaque opération asynchrone repose sur l'achèvement de la précédente, conduisant à des rappels profondément imbriqués.

L'enfer des rappels se produit généralement lorsque vous avez plusieurs tâches asynchrones qui doivent être exécutées en séquence, et chaque tâche dépend du résultat de la précédente. À mesure que de nouvelles tâches sont ajoutées, le code devient de plus en plus en retrait et plus difficile à suivre, ce qui entraîne une structure désordonnée et complexe.

getData((data) => {
  processData(data, (processedData) => {
    saveData(processedData, (savedData) => {
      notifyUser(savedData, () => {
        console.log("All tasks complete!");
      });
    });
  });
});

Here, each task is dependent on the previous one, leading to multiple levels of indentation and difficult-to-follow logic. If there’s an error at any point, handling it properly becomes even more complex.

To rescue you from callback hell, modern JavaScript provides solutions like:

1. Promises – which flatten the code and make it more readable.

2. Async/Await – which simplifies chaining asynchronous operations and makes the code look synchronous.

Promises

A Promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and its result. It’s like a promise you make in real life: something may not happen immediately, but you either fulfill it or fail to fulfill it.

In JavaScript, Promises allow you to write asynchronous code in a cleaner way, avoiding callback hell. A Promise is created using the new Promise syntax, and it takes a function (which is constructor function) with two parameters: resolve (if the task is successful) and reject (if it fails).

const myPromise = new Promise((resolve, reject) => {
  // Simulating an asynchronous task like fetching data
  const success = true; // Change to false to simulate failure

  setTimeout(() => {
    if (success) {
      resolve("Task completed successfully!"); // Success
    } else {
      reject("Task failed!"); // Failure
    }
  }, 2000); // 2-second delay
});

Here:

• The promise simulates a task that takes 2 seconds.

• If the task is successful (success is true), it calls resolve with a message.

• If it fails (success is false), it calls reject with an error message.

How to handle a Promise

To handle the result of a Promise, we use two methods:

• .then() for a successful result (when the Promise is fulfilled).

• .catch() for an error (when the Promise is rejected).
  

myPromise
  .then((message) => {
    console.log(message); // "Task completed successfully!" (if resolve was called)
  })
  .catch((error) => {
    console.log(error); // "Task failed!" (if reject was called)
  });

Chaining in Promises

Promises allow you to chain asynchronous operations using the .then() method, which makes the code more linear and easier to follow. Each .then() represents a step in the asynchronous process. The .catch() method allows you to centralize error handling at the end of the promise chain, making it more organized.

fetchData()
  .then(data => processData1(data))
  .then(result1 => processData2(result1))
  .then(result2 => processData3(result2))
  .catch(error => handleError(error));

Promises promote a more readable and maintainable code structure by avoiding deeply nested callbacks. The chaining and error-handling mechanisms contribute to a clearer and more organized codebase.

// Callback hell
fetchData1(data1 => {
  processData1(data1, result1 => {
    fetchData2(result1, data2 => {
      processData2(data2, result2 => {
        fetchData3(result2, data3 => {
          processData3(data3, finalResult => {
            console.log("Final result:", finalResult);
          });
        });
      });
    });
  });
});

// Using Promises
fetchData1()
  .then(result1 => processData1(result1))
  .then(data2 => fetchData2(data2))
  .then(result2 => processData2(result2))
  .then(data3 => fetchData3(data3))
  .then(finalResult => {
    console.log("Final result:", finalResult);
  })
  .catch(error => handleError(error));

finally Method

The finally method is used to execute code, regardless of whether the Promise is resolved or rejected.

myPromise
  .then((data) => {
    console.log("Data:", data);
  })
  .catch((error) => {
    console.error("Error:", error);
  })
  .finally(() => {
    console.log("Finally block"); // This runs no matter what
  });

If the promise is fulfilled (i.e., it successfully gets data), the .then() method will be executed. On the other hand, if the promise encounters an error, the .catch() method will be called to handle the error. However, the .finally() method has no such condition—it will always be executed, regardless of whether the promise was resolved or rejected. Whether the .then() or .catch() is triggered, the .finally() block will definitely run at the end.

It's useful for cleaning up resources, stopping loading indicators, or doing something that must happen after the promise completes, regardless of its result.

JavaScript Promise Methods

JavaScript provides several Promise methods that make working with asynchronous tasks more flexible and powerful. These methods allow you to handle multiple promises, chain promises, or deal with various outcomes in different ways

Method	Description
all (iterable)	Waits for all promises to be resolved or any one to be rejected.
allSettled (iterable)	Waits until all promises are either resolved or rejected.
any (iterable)	Returns the promise value as soon as any one of the promises is fulfilled.
race (iterable)	Waits until any of the promises is resolved or rejected.
reject (reason)	Returns a new promise object that is rejected for the given reason.
resolve (value)	Returns a new promise object that is resolved with the given value.

Promise.all() - Parallel Execution

You can use Promise.all() to execute multiple asynchronous operations concurrently and handle their results collectively.

This method takes an array of promises and runs them in parallel which returns a new Promise. This new Promise fulfils with an array of resolved values when all the input Promises have fulfilled, or rejects with the reason of the first rejected Promise.

Copy code
const promise1 = Promise.resolve(10);
const promise2 = Promise.resolve(20);
const promise3 = Promise.resolve(30);

Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values); // [10, 20, 30]
});

If one of the promises rejects, the whole Promise.all() will reject:

const promise1 = Promise.resolve(10);
const promise2 = Promise.reject("Error!");

Promise.all([promise1, promise2])
  .then((values) => {
    console.log(values);
  })
  .catch((error) => {
    console.log(error); // "Error!"
  });

Promise.allSettled()

This method takes an array of promises and returns a new promise after all of them finish their execution, whether they succeed or fail. Unlike Promise.all(), it doesn't fail if one promise fails. Instead, it waits for all the promises to complete and gives you an array of results that show if each one succeeded or failed.

It’s useful when you want to know the result of every promise, even if some fail.

const promise1 = Promise.resolve("Success!");
const promise2 = Promise.reject("Failure!");

Promise.allSettled([promise1, promise2]).then((results) => {
  console.log(results);
  // [{ status: 'fulfilled', value: 'Success!' }, { status: 'rejected', reason: 'Failure!' }]
});

Promise.any()

Promise.any() takes an array of promises and returns a promise that resolves as soon as any one promise resolves. If all input Promises are rejected, then Promise.any() rejects with an AggregateError containing an array of rejection reasons.

It’s useful when you need just one successful result from multiple promises.

const promise1 = fetchData1();
const promise2 = fetchData2();
const promise3 = fetchData3();

Promise.any([promise1, promise2, promise3])
  .then((firstFulfilledValue) => {
    console.log("First promise fulfilled with:", firstFulfilledValue);
  })
  .catch((allRejectedReasons) => {
    console.error("All promises rejected with:", allRejectedReasons);
  });

Promise.race()

This method takes an array of promises and returns a new promise that resolves or rejects as soon as the first promise settles (either resolves or rejects).

It’s useful when you want the result of the fastest promise, ignoring the others.

const promise1 = new Promise((resolve) => setTimeout(resolve, 100, "First"));
const promise2 = new Promise((resolve) => setTimeout(resolve, 200, "Second"));

Promise.race([promise1, promise2]).then((value) => {
  console.log(value); // "First" (because promise1 resolves first)
});

Promise.resolve()

This method immediately resolves a promise with a given value. It’s useful for creating a promise that you already know will be fulfilled.

const resolvedPromise = Promise.resolve("Resolved!");
resolvedPromise.then((value) => {
  console.log(value); // "Resolved!"
});

Promise.reject()

This method immediately rejects a promise with a given error or reason. It’s useful for creating a promise that you know will fail.

const rejectedPromise = Promise.reject("Rejected!");
rejectedPromise.catch((error) => {
  console.log(error); // "Rejected!"
});

I hope you have no confusion regarding promises after carefully reading this explanation of promises. Now, it’s time to move on Async/Await.

Async/Await

async / await is a modern way to handle asynchronous operations in JavaScript, introduced in ES2017 (ES8). It’s built on top of Promises but provides a cleaner and more readable syntax, making asynchronous code look and behave more like synchronous code. This makes it easier to understand and debug.

How async and await Work

When you declare a function with the async keyword, it always returns a Promise. Even if you don't explicitly return a promise, JavaScript wraps the return value inside a resolved promise.

The await keyword can only be used inside an async function. It makes JavaScript wait for the promise to resolve before moving on to the next line of code. It “pauses” the execution of the function, allowing you to handle asynchronous tasks more sequentially, just like synchronous code.

Let’s look at a simple example to see how this works:

async function fetchData() {
  try {
    const response = await fetch("https://api.example.com/data");
    const data = await response.json();
    console.log("Data:", data);
  } catch (error) {
    console.error("Error fetching data:", error);
  } finally {
    console.log("Fetch operation complete");
  }
}

fetchData();

How it works:

1. async function: Since fetchData function is marked as async, that means it returns a promise.

2. await fetch(): The await pauses the execution of the function until the promise returned by fetch() resolves. It then continues with the next line after the promise is resolved.

3. try/catch: We use a try/catch block to handle any potential errors during the async operation.

4. finally: Regardless of success or failure, the finally block will execute.

Why Use async / await?

With async/await, your code becomes more readable and flows more naturally. This makes it easier to follow the logic, especially when dealing with multiple asynchronous operations.

Compare this to handling promises with .then():

fetch("https://api.example.com/data")
  .then(response => response.json())
  .then(data => {
    console.log("Data:", data);
  })
  .catch(error => {
    console.error("Error fetching data:", error);
  })
  .finally(() => {
    console.log("Fetch operation complete");
  });

The async/await version looks cleaner and easier to understand. async/await helps avoid the nesting of callbacks or .then() chains, making your code more linear and easier to follow.

Example with Multiple await

You can also handle multiple asynchronous tasks in a sequence without needing to chain promises.

async function processOrders() {
  const user = await getUserDetails();  // Waits for user details
  const orders = await getOrders(user.id);  // Waits for orders
  console.log("Orders:", orders);
}

processOrders();

In this example, the function waits for each task to finish before moving on to the next, just like how synchronous code would behave.

Parallel Execution with Promise.all() and async/await

If you want to perform multiple asynchronous operations at the same time (in parallel), you can still use Promise.all() with async/await:

async function getAllData() {
  const [user, orders] = await Promise.all([getUserDetails(), getOrders()]);
  console.log("User:", user);
  console.log("Orders:", orders);
}

Here, both getUserDetails() and getOrders() run simultaneously, and the function waits for both to finish before logging the results.

Conclusion

In JavaScript, handling asynchronous operations has evolved over time, offering different tools to make code more manageable and efficient. Callbacks were the first approach, but as the complexity of code grew, they often led to issues like "callback hell." Promises came next, providing a cleaner, more structured way to manage async tasks with .then() and .catch(), improving readability and reducing nesting.

Finally, async/await was introduced as a modern syntax built on promises, making asynchronous code look more like synchronous code. It simplifies the process even further, allowing for easier-to-read, more maintainable code. Each of these techniques plays an important role in JavaScript, and mastering them helps you write more efficient, clear, and robust code.

Understanding when to use each method—callbacks for simple tasks, promises for structured handling, and async/await for readable, scalable async code—will empower you to make the best choices for your projects.

My Last Words

I used to get confused by the concept of promises, especially the different methods of promises. Callbacks were a big challenge for me because the syntax always seemed very confusing. So, I read various sources online, including chatbots, to come up with this description. To be honest, chatbots don’t always provide a straight and understandable answer. So, I didn’t just copy and paste from different places—I simplified everything so that it can serve as a clear note for me and for anyone else who has difficulty grasping these concepts. I hope this note leaves you with zero confusion.

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