In TypeScript, you can use async and await with a Promise to simplify asynchronous code and make it more readable.

What is a Promise?

A promise in TypeScript is an object representing the eventual completion or failure of an asynchronous operation. It acts as a placeholder for a value that may be available now, in the future, or never.

Syntax:

const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation or task
  // If successful, call resolve with the result
  // If there's an error, call reject with the reason
});

A promise is typically created using the Promise constructor, which takes a callback function with resolve and reject parameters. The asynchronous operation is performed inside this callback.

Why Use Async/Await with a Promise in TypeScript?

The async/await syntax in TypeScript provides a more concise and readable way to work with promises. It makes asynchronous code look and behave like synchronous code, making it easier to understand.

Using async/await with promises helps avoid the callback hell (also known as the pyramid of doom) and makes error handling more straightforward.

Steps to Use async/await with a Promise in TypeScript

1. Create an Asynchronous Function

Use the async keyword before a function declaration to indicate that it contains asynchronous code.

async function myAsyncFunction(): Promise<void> {
  // Asynchronous code with await keyword
  try {
    const result = await myPromiseFunction();
    // Process result
  } catch (error) {
    // Handle errors
  }
}

2. Use the await Keyword

Inside the asynchronous function, use the await keyword before calling a promise. This pauses the execution of the function until the promise is resolved or rejected.

async function myAsyncFunction(): Promise<void> {
  try {
    const result = await myPromiseFunction();
    // Process result
  } catch (error) {
    // Handle errors
  }
}

Example

Here, fetchData returns a promise, and fetchDataAsync is an asynchronous function that uses await to wait for the promise to be resolved. The try/catch block handles any errors that might occur during the asynchronous operation.

// Function returning a promise
function fetchData(): Promise<string> {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("Async data has been fetched!");
    }, 2000);
  });
}

// Async function using await to handle the promise
async function fetchDataAsync(): Promise<void> {
  try {
    const result = await fetchData();
    console.log(result); // Output: Async data has been fetched!
  } catch (error) {
    console.error("Error fetching data:", error);
  }
}

// Call the async function
fetchDataAsync();

Output:

Async data has been fetched!

FAQs – Use Async/Await With Promise in TypeScript

What is the main advantage of using async/await over traditional promise chaining?

The main advantage is improved readability and maintainability. async/await makes asynchronous code look like synchronous code, which is easier to read and debug.

Can we use await outside of an async function?

No, the await keyword can only be used inside an async function. Using it outside an async function will result in a syntax error.

How does async/await improve error handling in asynchronous code?

async/await allows you to use try/catch blocks for error handling, similar to synchronous code. This provides a more consistent and readable way to handle errors compared to promise chaining with .catch().

What happens if an async function does not explicitly return a promise?

If an async function does not explicitly return a promise, it implicitly returns a promise that resolves to the value returned by the function or rejects with an error thrown inside the function.

Can async/await be used with non-promise-based asynchronous operations?

No, async/await is designed to work with promises. Non-promise-based asynchronous operations need to be converted to promises to be used with async/await.