Promise.resolve()

Introduction

Promise.resolve() creates a Promise object that is already resolved with the specified value. If the value passed to Promise.resolve() is a Promise object, the method simply returns the Promise object itself. If the value is a thenable object, Promise.resolve() converts it into a Promise and resolves it with the value returned by the then() method.

Examples

Here are some examples to illustrate the usage of Promise.resolve():

Example 1: Resolving a value with Promise.resolve()

const promise = Promise.resolve("Hello, world!");
promise.then((value) => console.log(value)); // Output: 'Hello, world!'

Example 2: Resolving a Promise with Promise.resolve()

const originalPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve("Original Promise Resolved");
  }, 2000);
});

const resolvedPromise = Promise.resolve(originalPromise);
resolvedPromise.then((value) => console.log(value)); // Output: 'Original Promise Resolved' after 2 seconds

Detailed explanation

Syntax

Promise.resolve(value);

The Promise.resolve() method takes one parameter, which is the value to be resolved. The value can be any JavaScript object, including a Promise or thenable object.

Return value

The Promise.resolve() method returns a Promise object that is resolved with the specified value. If the value passed to Promise.resolve() is already a Promise object, the method simply returns the Promise object itself. If the value is a thenable object, Promise.resolve() converts it into a Promise and resolves it with the value returned by the then() method.

Limitations

Promise.resolve() can only resolve one value at a time. If you need to resolve multiple values, you can use Promise.all() instead.

Tips and tricks

Promise.resolve() can be used to simplify the code when dealing with asynchronous operations. For example, if you want to create a Promise that resolves with a specific value, you can simply use Promise.resolve(value) instead of creating a new Promise object.

Promise.resolve() can be used in the following scenarios:

• Resolving a value with a Promise
• Converting a thenable object to a Promise
• Simplifying the code when dealing with Promises

When using Promise.resolve(), it's best to ensure that the value passed as a parameter is not a Promise object unless intended. Also, it's important to handle errors appropriately in the then() method or with a catch() method.

Promise.resolve() can be used in conjunction with Promise.all() to create an array of Promises that are resolved with specific values.

ECMAScript specification

Promise.resolve() was introduced in ECMAScript 6 (ES6) and is part of the core language specification. It is widely supported in modern browsers and Node.js versions.

See ECMAScript specification

Exercises

1. Create a function named getUserDetails that accepts a parameter userId.
2. Inside the function, create a Promise that resolves with a user object containing details such as name, email, and address for the given userId.
3. Call getUserDetails with a valid userId and log the resolved value to the console.
4. Call getUserDetails with an invalid userId and handle the rejection by logging an error message to the console.

function getUserDetails(userId) {
  return new Promise((resolve, reject) => {
    // Assume this is an API call that fetches user details for the given userId
    const user = {
      id: userId,
      name: "John Doe",
      email: "johndoe@example.com",
      address: "123 Main Street",
    };

    if (userId > 0) {
      resolve(user);
    } else {
      reject(new Error("Invalid userId"));
    }
  });
}

// Call getUserDetails with a valid userId
getUserDetails(123)
  .then((user) => {
    console.log("User details:", user);
  })
  .catch((error) => {
    console.error(error);
  });

// Call getUserDetails with an invalid userId
getUserDetails(-1)
  .then((user) => {
    console.log("User details:", user);
  })
  .catch((error) => {
    console.error(error);
  });