If you have been writing JavaScript code for a while, you must be familiar with asynchronous programming. Promise is the core of modern JS asynchronous processing, while async/await is just the syntax sugar of Promise, which makes the asynchronous code look more like synchronous code, improving readability and maintenance. However, many people do not fully understand its relationship with Promise when using async/await, which leads to some common mistakes or misunderstandings.

Let’s take a look at a few key points to help you better understand and use async/await and promises to work together.

The Async function automatically returns a promise

When you define an async function, JavaScript will automatically wrap its return value into a promise. This is very important because it determines how you handle the result of this function.

for example:

 async function getData() {
  return 'hello';
}

The above function is actually equivalent to:

 function getData() {
  return Promise.resolve('hello');
}

So if you throw an error from the async function, it will also become a rejected Promise:

 async function getError() {
  throw new Error('something went wrong');
}

This means that when you call the async function, you must use .then() or await to get the result , or use try/catch to catch the error.

Await must appear inside the Async function

This is a mistake that many newbies are prone to make: directly use await in global scope or ordinary functions, and the result is an error.

 // An error occurred!
const data = await fetchData();

await can only be used in async functions. If you want to use await in top-level scope (such as in modules), it depends on whether the environment supports top-level await (supported from ES2022), otherwise you can only handle it through .then() and .catch() .

Multiple asynchronous tasks can be executed in parallel

While await makes the code seem to be executed sequentially, if you have several asynchronous operations that are not dependent on each other, you can execute them in parallel instead of one after another.

For example:

 async function fetchBoth() {
  const a = await fetchA(); // Wait for the first to complete const b = await fetchB(); // Wait for the second}

This code is serial and inefficient. We can change it to parallel through Promise.all() :

 async function fetchBoth() {
  const [a, b] = await Promise.all([fetchA(), fetchB()]);
}

In this way, the two requests will be initiated at the same time, waiting for the slowest one to end.

Notes:

• If one of them fails, the entire Promise.all() will reject immediately.
• If you want to continue execution even if there is a failure, you can use Promise.allSettled() .

Error handling cannot be ignored

When using async/await , the easiest thing to ignore is error handling. You can use try/catch to wrap the await expression to catch exceptions:

 async function loadData() {
  try {
    const data = await fetchData();
    console.log(data);
  } catch (error) {
    console.error('Failed to load data:', error);
  }
}

But sometimes you will see this writing:

 const data = await fetchData().catch(err => {
  console.error(err);
});

Although this can also handle errors, it will make the logic scattered. If there are subsequent operations that rely on data , additional judgments may be required to determine whether data exists.

Therefore, it is recommended to use the try/catch structure first to clearly express the paths of success and failure.

Basically that's it. Async/Await does simplify the use of Promise, but to really use it well, you still need to understand the working mechanism behind it. After mastering these details, you will find that writing asynchronous code has become more intuitive and controllable.

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