Promise.race in JavaScript with Practical Examples

Promise.race Exercise:

Write a JavaScript program that demonstrates the use of Promise.race to return the first completed Promise.

Solution-1:

Code:

// Create three promises with varying delays
const promise1 = new Promise((resolve) => {
  setTimeout(() => resolve('Promise 1 resolved!'), 3000); // Resolves after 3 seconds
});

const promise2 = new Promise((resolve) => {
  setTimeout(() => resolve('Promise 2 resolved!'), 2000); // Resolves after 2 seconds
});

const promise3 = new Promise((resolve) => {
  setTimeout(() => resolve('Promise 3 resolved!'), 1000); // Resolves after 1 second
});

// Use Promise.race to get the first resolved promise
Promise.race([promise1, promise2, promise3]).then((result) => {
  console.log(result); // Output: Promise 3 resolved!
});

Output:

"Promise 3 resolved!"

Explanation:

• Three promises are created with different delays (1, 2, and 3 seconds).
• 'Promise.race' returns the result of the first promise that resolves, which is 'promise3' in this case.
• The resolved value of 'promise3' is logged to the console.

Solution-2:

Code:

// Create promises for simulating API calls with varying delays
const apiCall1 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('API Call 1 Success'), 1500); // Resolves after 1.5 seconds
});

const apiCall2 = new Promise((resolve, reject) => {
  setTimeout(() => resolve('API Call 2 Success'), 1000); // Resolves after 1 second
});

const apiCall3 = new Promise((resolve, reject) => {
  setTimeout(() => reject('API Call 3 Failed'), 2000); // Rejects after 2 seconds
});

// Use Promise.race to handle the first completed promise
Promise.race([apiCall1, apiCall2, apiCall3])
  .then((result) => {
    console.log(result); // Output: API Call 2 Success
  })
  .catch((error) => {
    console.error(error); // Handles the rejection if it happens first
  });

Output:

"API Call 2 Success"

Explanation:

• Three promises simulate API calls with different completion times (1, 1.5, and 2 seconds).
• 'Promise.race' resolves or rejects with the result of the first completed promise.
• In this case, 'apiCall2' resolves first, so its success message is logged.
• If a rejection happens first, the `catch` block handles it.

See the Pen promises-and-async-await-exercise-15 by w3resource (@w3resource) on CodePen.

For more Practice: Solve these Related Problems:

• Write a JavaScript function that uses Promise.race to return the result of the fastest among several asynchronous operations.
• Write a JavaScript function that implements a timeout mechanism for an API call using Promise.race.
• Write a JavaScript program that races a delayed Promise against an immediate one and returns the quickest result using Promise.race.
• Write a JavaScript function that employs Promise.race to select between multiple simulated network responses and logs the fastest one.

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