Creating promises in Javascript

Introduction

Promises help you handle tasks that take time, like loading data. They let your program wait for the task to finish without stopping everything.

When you want to load data from the internet and wait for it to arrive.

When you need to do something after a timer finishes.

When you want to run a task that might succeed or fail and handle both cases.

When you want to avoid freezing the screen while waiting for a task.

Syntax

Javascript

const myPromise = new Promise((resolve, reject) => {
  // do something asynchronous
  const success = true; // example condition
  if (success) {
    resolve(result); // task finished successfully
  } else {
    reject(error); // task failed
  }
});

resolve is called when the task finishes well.

reject is called when the task fails.

Examples

This promise immediately finishes successfully with the message 'Done!'.

Javascript

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

This promise waits 1 second, then finishes successfully.

Javascript

const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Waited 1 second');
  }, 1000);
});

This promise finishes with an error if error is true.

Javascript

const promise = new Promise((resolve, reject) => {
  const error = true;
  if (error) {
    reject('Something went wrong');
  } else {
    resolve('All good');
  }
});

Sample Program

This program creates a promise that finishes successfully. It then prints the success message.

Javascript

const myPromise = new Promise((resolve, reject) => {
  const success = true;
  if (success) {
    resolve('Task completed successfully');
  } else {
    reject('Task failed');
  }
});

myPromise
  .then(message => {
    console.log('Success:', message);
  })
  .catch(error => {
    console.log('Error:', error);
  });

Important Notes

Promises always run the code inside immediately when created.

Use then to handle success and catch to handle errors.

Promises help keep your program responsive by not blocking other code.

Summary

Promises represent tasks that finish later with success or failure.

You create a promise with new Promise((resolve, reject) => { ... }).

Call resolve when done well, reject when there is an error.

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