Javascript

Understanding JavaScript Promise.withResolvers Method

In modern JavaScript development, asynchronous programming is crucial for building responsive and efficient applications. Promises are a powerful tool for managing asynchronous operations, providing a cleaner and more readable alternative to nested callbacks. With the introduction of the Promise.withResolvers method, JavaScript developers now have even more flexibility and control over the resolution of promises. In this blog post, we’ll explore what Promise.withResolvers is, how it works, and provide practical examples to demonstrate its usage.

What does promise.withResolvers?

Promise.withResolvers is a method introduced in ECMAScript 2021 (ES12) as part of the JavaScript language specification. It allows developers to create and handle promises using explicit resolver functions, providing a more structured approach to asynchronous programming.

The syntax for Promise.withResolvers is as follows:

let { promise, resolve, reject } = Promise.withResolvers();

How Does it Work?

The Promise.withResolvers method takes a single argument: a function that receives two parameters, resolve and reject. Inside this function, you can perform any asynchronous operations, such as fetching data from a server or reading a file. You then use the resolve function to fulfill the promise with a value when the operation is successful, or the reject function to reject the promise with a reason if an error occurs.

Practical Examples

Let’s dive into some practical examples to illustrate the usage of Promise.withResolvers:

Old Way:

Promise.withResolvers((resolve, reject) => {
  setTimeout(() => {
    const randomNumber = Math.random();
    if (randomNumber < 0.5) {
      resolve(randomNumber);
    } else {
      reject(new Error('Random number is too high'));
    }
  }, 1000);
});

New Way:

async function* readableTableToAsyncIterable(stream) {
  let { promise, resolve, reject } = Promise.withResolvers();
  stream.on("error", (error) => reject(error));
  stream.on("end", () => resolve());
  stream.on("readable", () => resolve());

  while (stream.readable) {
    await promise;
    let chunk;
    while ((chunk = stream.read())) {
      yield chunk;
    }
    ({ promise, resolve, reject } = Promise.withResolvers());
  }
}

Example 1: Fetching Data from an API

async function fetchDataFromAPI(url) {
  let { promise, resolve, reject } = Promise.withResolvers();

  fetch(url)
    .then(response => {
      if (!response.ok) {
        throw new Error('Failed to fetch data');
      }
      return response.json();
    })
    .then(data => {
      resolve(data);
    })
    .catch(error => {
      reject(error);
    });

  return promise;
}

// Usage example:
fetchDataFromAPI('https://api.example.com/data')
  .then(data => {
    console.log('Data fetched successfully:', data);
  })
  .catch(error => {
    console.error('Error fetching data:', error);
  });
  • We define an async function fetchDataFromAPI that takes a URL as a parameter.
  • Inside this function, we create a new promise, resolve, and reject using Promise.withResolvers().
  • We use fetch to make a request to the specified URL.
  • If the response is successful (status code 200-299), we parse the JSON response and resolve the promise with the data.
  • If there’s an error during the fetch or the response is not successful, we reject the promise with an error.
  • Finally, we return the promise from the function.

You can then use fetchDataFromAPI to fetch data from any API endpoint and handle the response using .then() and .catch() as shown in the usage example.

Example 2: Simulating a Delayed Operation

Promise.withResolvers((resolve, reject) => {
  setTimeout(() => {
    const randomNumber = Math.random();
    if (randomNumber < 0.5) {
      resolve(randomNumber);
    } else {
      reject(new Error('Random number is too high'));
    }
  }, 1000);
});

In this example, we use Promise.withResolvers to simulate a delayed operation (e.g., fetching data from a database). We use setTimeout to introduce a delay of 1000 milliseconds, then generate a random number. If the random number is less than 0.5, we fulfill the promise with the random number; otherwise, we reject the promise with an error.

Promise resolve without value

function examplePromise() {
  let { promise, resolve, reject } = Promise.withResolvers();

  // Simulate an asynchronous operation
  setTimeout(() => {
    // Resolve the promise without providing a value
    resolve();
  }, 2000);

  return promise;
}

// Using the promise
examplePromise()
  .then(() => {
    console.log('Promise resolved without a value');
  })
  .catch(error => {
    console.error('Promise rejected:', error);
  });
  • We define a function examplePromise that creates a new promise using Promise.withResolvers().
  • Inside this function, we simulate an asynchronous operation using setTimeout.
  • After 2 seconds (2000 milliseconds), we call resolve() without providing any value.
  • Finally, we return the promise from the function.

When using the examplePromise function:

  • The then method is called when the promise is resolved, and it logs “Promise resolved without a value” to the console.
  • Since we didn’t provide a value to the resolve function, the fulfillment value of the promise is undefined.
  • There’s no catch method in this example, but you can add one to handle potential errors that may occur during the asynchronous operation.

Promise resolve with value

function examplePromiseWithValue() {
  let { promise, resolve, reject } = Promise.withResolvers();

  // Simulate an asynchronous operation
  setTimeout(() => {
    // Resolve the promise with a value
    resolve('Hello, world!');
  }, 2000);

  return promise;
}

// Using the promise
examplePromiseWithValue()
  .then(value => {
    console.log('Promise resolved with value:', value);
  })
  .catch(error => {
    console.error('Promise rejected:', error);
  });
  • We define a function examplePromiseWithValue that creates a new promise using Promise.withResolvers().
  • Inside this function, we simulate an asynchronous operation using setTimeout.
  • After 2 seconds (2000 milliseconds), we call resolve('Hello, world!') to resolve the promise with the value 'Hello, world!'.
  • Finally, we return the promise from the function.

When using the examplePromiseWithValue function:

  • The then method is called when the promise is resolved, and it logs “Promise resolved with value: Hello, world!” to the console.
  • The value passed to the resolve function ('Hello, world!') is captured as the fulfillment value of the promise.
  • There’s no catch method in this example, but you can add one to handle potential errors that may occur during the asynchronous operation.

What does promise.resolve do?

The Promise.resolve() method creates a new Promise object that is resolved with the given value or Promise-like object. Here’s how it works:

  1. If the value passed to Promise.resolve() is a promise (i.e., it has a then method), Promise.resolve() returns that promise unchanged.
  2. If the value passed to Promise.resolve() is not a promise, it creates a new promise that is immediately resolved with the provided value.

Here’s an example to illustrate both cases:

// Example 1: Resolving with a non-promise value
const nonPromiseValue = 42;
const promise1 = Promise.resolve(nonPromiseValue);

promise1.then(value => {
  console.log(value); // Output: 42
});

// Example 2: Resolving with a promise
const promise2 = Promise.resolve(new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Resolved inside inner promise');
  }, 1000);
}));

promise2.then(value => {
  console.log(value); // Output: Resolved inside inner promise
});

Resolve promise without await?

You can resolve a promise without using await by simply attaching a then handler to the promise. Here’s an example:

function examplePromise() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve('Promise resolved without await');
    }, 2000);
  });
}

// Resolving the promise without await
examplePromise().then(value => {
  console.log(value); // Output: Promise resolved without await
});

console.log("This message will be printed immediately.");

In this example, examplePromise() creates a new promise that resolves with the string 'Promise resolved without await' after a delay of 2 seconds. Instead of using await, we attach a then handler to the promise returned by examplePromise(). When the promise is resolved, the then handler is called with the resolved value, and it logs the value to the console.

The advantage of using await is that it allows you to write asynchronous code in a synchronous-looking style, making it easier to understand and maintain. However, if you prefer to handle promise resolution asynchronously using then, you can do so as demonstrated in this example.

Is promise legally binding?

Promises in JavaScript, or any other programming language, are not legally binding agreements in the legal sense. They are constructs used for handling asynchronous operations in code.

In JavaScript, a Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It allows you to write asynchronous code in a more readable and manageable way. Promises themselves do not have any legal implications or enforceable contracts.

Legal agreements, on the other hand, are formal contracts or understandings between parties that are typically governed by legal principles and may have legal consequences if breached. These agreements are typically written in natural language, often with specific legal terminology, and may require signatures or other forms of authentication to be legally binding.

While you might use promises in your code to represent certain agreements or actions, they are entirely within the realm of programming and do not have any inherent legal significance. Any legal agreements between parties must be established and enforced through appropriate legal channels and mechanisms.

 

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