Creating promises in Javascript - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this promise code?

Consider the following JavaScript code that creates and uses a promise. What will be printed to the console?

Javascript

const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve('Done'), 100);
});

promise.then(result => console.log(result));

A undefined

B Done

C Error

D Promise { <pending> }

💡 Hint

Remember that the promise resolves after 100 milliseconds and then calls the .then() callback.

The promise resolves after 100ms with the value 'Done'. The .then() method receives this value and logs it.

❓ Predict Output

intermediate

What does this promise reject with?

Look at this code snippet. What will be the output when the promise is rejected?

Javascript

const promise = new Promise((resolve, reject) => {
  reject('Failed');
});

promise.catch(error => console.log(error));

A Failed

B undefined

C Error

D Promise { <rejected> }

💡 Hint

The reject function sends the reason to the catch handler.

The promise is immediately rejected with the string 'Failed'. The catch handler logs this string.

❓ Predict Output

advanced

What is the output of this chained promise?

Analyze the following code. What will be printed to the console?

Javascript

new Promise((resolve, reject) => {
  resolve(5);
})
.then(value => value * 2)
.then(value => { throw new Error('Oops'); })
.catch(error => 'Caught: ' + error.message)
.then(value => console.log(value));

A undefined

B 10

C Error: Oops

D Caught: Oops

💡 Hint

Notice the error thrown and how the catch handles it.

The promise resolves with 5, then doubles to 10, then throws an error. The catch handles the error and returns a string, which is logged.

❓ Predict Output

advanced

What will this promise output after 1 second?

What will be printed to the console after running this code?

Javascript

function delay(ms) {
  return new Promise(resolve => setTimeout(() => resolve('Waited'), ms));
}

delay(1000).then(msg => console.log(msg));

A undefined

B Promise { <pending> }

C Waited

D Error

💡 Hint

The promise resolves after the given milliseconds with the string 'Waited'.

The delay function returns a promise that resolves after 1 second with 'Waited'. The then callback logs it.

❓ Predict Output

expert

What is the output of this nested promise code?

Examine this code carefully. What will be printed to the console?

Javascript

new Promise((resolve, reject) => {
  resolve('Start');
})
.then(value => {
  return new Promise((res, rej) => {
    setTimeout(() => res(value + ' End'), 50);
  });
})
.then(result => console.log(result));

A Start End

B Start

C undefined

D Error

💡 Hint

The first then returns a promise that resolves after 50ms with a modified string.

The first promise resolves with 'Start'. The then returns a new promise that appends ' End' after 50ms. The final then logs 'Start End'.