Use once await to wait for multiple asynchronous processes.

One of the strengths of JavaScript is its ability to perform asynchronous operations to avoid processes waiting for each other. In practical applications, asynchronous operations are often used for processes whose waiting time depends on external factors, such as network connection, storage speed, etc. Here are some examples of asynchronous operations:

• Database query
• Get data from other websites (HTTP request)
• Read and write files
• Send email

Execution of asynchronous functions

In JavaScript, asynchronous operations are usually implemented using functions. A function is a set of blocks of code that perform a specific task, such as an addition function to calculate a sum and a shutdown function to shut down the computer (maybe there is such a function?).

Define function

To define an asynchronous function, just add the async keyword when the function is declared, and the rest is the same as an ordinary function.

For example, we define a function to send emails asynchronously:

async function kirimEmail(tujuan, judul, isi) {
  // 發(fā)送郵件
  // ...
}

Or use arrow functions:

const kirimEmail = async (tujuan, judul, isi) => {
  // 發(fā)送郵件
  // ...
}

Call function

When the above function is called, it will automatically execute asynchronously, which means there will be no waiting for each other.

For example:

kirimEmail('contoh1@email.com', 'Tes 1 Email', 'Halo. Ini saya lagi ngetes.');
kirimEmail('contoh2@email.com', 'Tes 2 Email', 'Halo. Ini saya lagi ngetes.');
kirimEmail('contoh3@email.com', 'Tes 3 Email', 'Halo. Ini saya lagi ngetes.');

In the above example, all processes that send emails are executed in sequence, but no one process is waited for to complete. Therefore, the next process will be started while the previous process is not completed and will not block each other.

If you need to get data or wait for an asynchronous process to complete, you can use the await keyword when calling a function.

For example:

await kirimEmail('contoh1@email.com', 'Tes 1 Email', 'Halo. Ini saya lagi ngetes.');
await kirimEmail('contoh2@email.com', 'Tes 2 Email', 'Halo. Ini saya lagi ngetes.');
await kirimEmail('contoh3@email.com', 'Tes 3 Email', 'Halo. Ini saya lagi ngetes.');

In this example, each process will wait for the previous process to complete. This usually happens when data needs to be fetched from one process for the next process, so one has to wait for the previous process to complete before fetching the required data.

Promise.all()

Using Promise.all, we can call and wait for multiple asynchronous functions at the same time.

Call function

An example of using Promise.all to call multiple asynchronous functions is as follows:

await Promise.all([
  kirimEmail('contoh1@email.com', 'Tes 1 Email', 'Halo. Ini saya lagi ngetes.'),
  kirimEmail('contoh2@email.com', 'Tes 2 Email', 'Halo. Ini saya lagi ngetes.'),
  kirimEmail('contoh3@email.com', 'Tes 3 Email', 'Halo. Ini saya lagi ngetes.'),
]);

In the above example, we wait for the completion of three sending mail processes, and the results will be returned in the form of an array regardless of success or failure.

Features

Promise.all has the following properties:

• The asynchronous function to be executed is placed in an array;
• All functions are executed as a whole, from startup to completion;
• Since they work as a whole, all processes will wait until completion, especially if await is used;
• If an error occurs in one of the processes, all processes will report an error.

Please refer to MDN's Promise.all documentation for more details.

Conclusion

Using Promise.all is very convenient, we only need to write await ? once, and it is also useful if we want to stop all processes if one of them errors. However, if we want to continue executing other processes even if one process fails, we will discuss this in the next article.

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