Creating promises in Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new Promise that resolves with 'Done'.

Javascript

const promise = new Promise(([1]) => {
  resolve('Done');
});

Drag options to blanks, or click blank then click option'

Adone

Breject

Cresolve

Dcallback

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'reject' instead of 'resolve' when fulfilling the promise.

Using a parameter name that is not 'resolve' or 'reject'.

The Promise constructor takes a function with 'resolve' and 'reject' parameters. Here, we use 'resolve' to fulfill the promise.

2fill in blank

medium

Complete the code to create a Promise that rejects with an error message.

Javascript

const promise = new Promise((resolve, [1]) => {
  [1]('Error occurred');
});

Drag options to blanks, or click blank then click option'

Areject

Bresolve

Cfail

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Calling 'resolve' instead of 'reject' to reject the promise.

Using incorrect parameter names.

The second parameter of the Promise executor function is 'reject', used to reject the promise with an error.

3fill in blank

hard

Fix the error in the Promise executor function parameter to correctly create a Promise.

Javascript

const promise = new Promise([1] => {
  resolve('Success');
});

Drag options to blanks, or click blank then click option'

Aresolve

B()

C(callback)

D(resolve, reject)

Attempts:

3 left

💡 Hint

Common Mistakes

Using a single parameter without parentheses.

Omitting the reject parameter.

The Promise executor function must have two parameters: resolve and reject, enclosed in parentheses.

4fill in blank

hard

Fill both blanks to create a Promise that resolves after 1 second with 'Done'.

Javascript

const promise = new Promise(([1], [2]) => {
  setTimeout(() => [1]('Done'), 1000);
});

Drag options to blanks, or click blank then click option'

Aresolve

Breject

Ccallback

Ddone

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names.

Calling reject instead of resolve inside setTimeout.

The Promise executor function takes 'resolve' and 'reject'. We call 'resolve' inside setTimeout to fulfill the promise after 1 second.

5fill in blank

hard

Fill all three blanks to create a Promise that resolves with the uppercase version of input after 500ms.

Javascript

function toUpperAsync(str) {
  return new Promise(([1], [2]) => {
    setTimeout(() => [1](str[3]), 500);
  });
}

Drag options to blanks, or click blank then click option'

Aresolve

Breject

C.toUpperCase()

D.toLowerCase()

Attempts:

3 left

💡 Hint

Common Mistakes

Using .toLowerCase() instead of .toUpperCase().

Calling reject instead of resolve.

Missing parentheses around parameters.

The Promise executor uses 'resolve' and 'reject'. We call resolve with str.toUpperCase() after 500ms.

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