Creating promises in Javascript - Try It Yourself

Creating promises

📖 Scenario: You are building a simple app that simulates checking if a user is logged in. This check takes some time, so you will use a promise to handle the waiting.

🎯 Goal: Create a promise that waits 2 seconds and then resolves with the message "User is logged in". Then display this message.

📋 What You'll Learn

Create a promise called loginCheck that waits 2 seconds before resolving

Use setTimeout inside the promise executor function

Resolve the promise with the exact string 'User is logged in'

Use loginCheck.then to get the resolved message

Print the resolved message using console.log

💡 Why This Matters

🌍 Real World

Promises are used in web apps to handle tasks like loading data from servers, waiting for user input, or running animations smoothly.

💼 Career

Understanding promises is essential for frontend and backend JavaScript developers to write clean, readable, and efficient asynchronous code.

1

Create the loginCheck promise

Create a promise called loginCheck using new Promise with an executor function that takes resolve and reject as parameters.

Javascript

// Your code here

Hint

Use new Promise((resolve, reject) => { }) to create the promise.

2

Add a 2-second delay and resolve the promise

Inside the loginCheck promise executor, use setTimeout to wait 2000 milliseconds, then call resolve with the string 'User is logged in'.

Javascript

const loginCheck = new Promise((resolve, reject) => {
  // Your code here
});

Hint

Use setTimeout(() => { resolve('User is logged in'); }, 2000) inside the promise.

3

Use then to get the resolved message

Use loginCheck.then with a function that takes a parameter message and inside it, call console.log(message).

Javascript

const loginCheck = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('User is logged in');
  }, 2000);
});

// Your code here

Hint

Use loginCheck.then(message => console.log(message)) to print the message.

4

Print the resolved message

Run the program and observe that after 2 seconds, the console prints User is logged in.

Javascript

const loginCheck = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('User is logged in');
  }, 2000);
});

loginCheck.then(message => {
  console.log(message);
});

// Your code here

Hint

Wait 2 seconds after running to see the message printed in the console.

Practice

1. What does the resolve function do inside a JavaScript Promise?

easy

A. It pauses the promise execution.

B. It marks the promise as failed.

C. It marks the promise as successfully completed.

D. It cancels the promise.

Solution

1. Step 1: Understand the role of resolve in promises

   The resolve function is called when the asynchronous task finishes successfully.

2. Step 2: Differentiate from reject

   reject is used for failure, so resolve must mean success.

3. Final Answer:

   It marks the promise as successfully completed. -> Option C

4. Quick Check:

   resolve = success [OK]

Hint: Resolve means success, reject means failure [OK]

Common Mistakes:

• Confusing resolve with reject
• Thinking resolve cancels the promise
• Believing resolve pauses execution

2. Which of the following is the correct syntax to create a new Promise in JavaScript?

easy

A. new Promise(resolve, reject) => { }

B. Promise.new((resolve, reject) => { })

C. new Promise { resolve, reject => }

D. new Promise((resolve, reject) => { })

Solution

1. Step 1: Recall the Promise constructor syntax

   The Promise constructor takes a single function with two parameters: resolve and reject, wrapped in parentheses.

2. Step 2: Check each option for correct syntax

   new Promise((resolve, reject) => { }) correctly uses new Promise((resolve, reject) => { }). Others have syntax errors.

3. Final Answer:

   new Promise((resolve, reject) => { }) -> Option D

4. Quick Check:

   Promise syntax = new Promise((resolve, reject) => { }) [OK]

Hint: Promise needs a function with (resolve, reject) inside parentheses [OK]

Common Mistakes:

• Missing parentheses around parameters
• Using wrong keyword order
• Incorrect arrow function syntax

3. What will be the output of this code?

const p = new Promise((resolve, reject) => {
  resolve('Done');
});
p.then(result => console.log(result));

medium

A. Done

B. undefined

C. Error

D. Promise { <pending> }

Solution

1. Step 1: Understand promise resolution

   The promise calls resolve('Done'), so it completes successfully with value 'Done'.

2. Step 2: Check what then does

   then receives the resolved value and logs it, so 'Done' is printed.

3. Final Answer:

   Done -> Option A

4. Quick Check:

   Resolved value logged = Done [OK]

Hint: Resolve value appears in then callback [OK]

Common Mistakes:

• Expecting undefined instead of resolved value
• Confusing promise object with resolved output
• Thinking promise logs automatically

4. Find the error in this Promise creation code:

const p = new Promise(resolve, reject) => {
  resolve('Success');
};

medium

A. The reject function is never called.

B. Arrow function syntax is incorrect for Promise constructor.

C. Promise constructor missing the new keyword.

D. No error, code is correct.

Solution

1. Step 1: Check Promise constructor usage

   The Promise constructor requires a single function argument wrapped in parentheses.

2. Step 2: Identify arrow function syntax error

   The code uses new Promise(resolve, reject) => { } which is invalid syntax. It should be new Promise((resolve, reject) => { }).

3. Final Answer:

   Arrow function syntax is incorrect for Promise constructor. -> Option B

4. Quick Check:

   Arrow function must be inside parentheses [OK]

Hint: Promise needs one function argument wrapped in parentheses [OK]

Common Mistakes:

• Not wrapping parameters in parentheses
• Omitting new keyword (not in this case)
• Using wrong arrow function syntax

5. You want to create a Promise that waits 1 second and then resolves with 'Done'. Which code correctly does this?

hard

A. new Promise((resolve) => { setTimeout(() => resolve('Done'), 1000); })

B. new Promise(resolve => setTimeout(resolve('Done'), 1000))

C. new Promise((resolve, reject) => { resolve('Done'); setTimeout(() => {}, 1000); })

D. new Promise(() => { setTimeout(() => resolve('Done'), 1000); })

Solution

1. Step 1: Understand how to delay resolution

   Use setTimeout to delay calling resolve by 1000 ms (1 second).

2. Step 2: Check each option for correct usage

   new Promise((resolve) => { setTimeout(() => resolve('Done'), 1000); }) correctly passes a function to Promise and calls resolve('Done') inside setTimeout. new Promise(resolve => setTimeout(resolve('Done'), 1000)) calls resolve immediately. new Promise((resolve, reject) => { resolve('Done'); setTimeout(() => {}, 1000); }) resolves immediately and delays nothing. new Promise(() => { setTimeout(() => resolve('Done'), 1000); }) does not receive resolve parameter.

3. Final Answer:

   new Promise((resolve) => { setTimeout(() => resolve('Done'), 1000); }) -> Option A

4. Quick Check:

   Delay resolve with setTimeout inside Promise [OK]

Hint: Wrap resolve call inside setTimeout function [OK]

Common Mistakes:

• Calling resolve immediately instead of inside setTimeout
• Not passing resolve parameter
• Placing setTimeout outside Promise