1. What is a Promise?

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. Think of it as a “placeholder” for a value that hasn’t arrived yet.

The 3 States of a Promise:

1. Pending: Initial state, neither fulfilled nor rejected.
2. Fulfilled (Resolved): The operation completed successfully.
3. Rejected: The operation failed (e.g., network error).

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2. Consuming Promises (.then, .catch, .finally)

When a promise is returned (like from a network request), we use handlers to deal with the result.

JavaScript

const myPromise = new Promise((resolve, reject) => {
    let success = true;
    if (success) {
        resolve("Data received!");
    } else {
        reject("Error: Connection lost.");
    }
});

myPromise
    .then((data) => console.log(data))    // Runs if Resolved
    .catch((err) => console.error(err))   // Runs if Rejected
    .finally(() => console.log("Done"));  // Runs always

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3. The Fetch API

fetch() is the modern way to make HTTP requests (API calls). It returns a Promise.

Crucial Note: A fetch() promise only rejects if there is a network error. It does not reject for 404 or 500 errors. You must check the response.ok property.

JavaScript

fetch('https://jsonplaceholder.typicode.com/posts/1')
    .then(response => {
        if (!response.ok) throw new Error("HTTP error " + response.status);
        return response.json(); // .json() also returns a promise!
    })
    .then(data => console.log(data))
    .catch(error => console.error("Request Failed:", error));

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4. Chaining Promises

One of the best features of Promises is that you can chain them. Each .then() returns a new promise, allowing you to process data in steps.

JavaScript

getUser(1)
    .then(user => getPosts(user.id))
    .then(posts => getComments(posts[0].id))
    .then(comments => console.log(comments))
    .catch(err => console.log(err)); // One catch for the whole chain!

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5. Handling Multiple Promises

Sometimes you need to trigger multiple requests at once. JavaScript provides static methods for this:

Method	Behavior
Promise.all([p1, p2])	Waits for all to succeed. If one fails, the whole thing fails immediately.
Promise.allSettled([p1, p2])	Waits for all to finish, regardless of success or failure. Returns an array of results.
Promise.race([p1, p2])	Returns the result of the first promise to settle (fastest one wins).
Promise.any([p1, p2])	Returns the first successful promise. If all fail, it throws an error.

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6. Avoiding “Callback Hell”

Before promises, we had nested functions inside functions. Promises flatten this structure, making the code much more readable.

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Technical Code Example: The Parallel Fetch

Let’s say you want to get a User and their Posts at the same time to save time.

JavaScript

const userReq = fetch('https://api.example.com/user/1');
const postReq = fetch('https://api.example.com/user/1/posts');

Promise.all([userReq, postReq])
    .then(responses => {
        // Convert both responses to JSON
        return Promise.all(responses.map(res => res.json()));
    })
    .then(([userData, postData]) => {
        console.log("User:", userData);
        console.log("Posts:", postData);
    })
    .catch(err => console.error("One of the requests failed", err));

In Simple words

1. Why Promises Matter

Modern applications constantly:

• Fetch data from servers
• Communicate with microservices
• Handle async workflows
• Manage failures gracefully

Promises provide a clean, predictable, and composable way to handle async code.

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2. The Problem with Callbacks

Callback Hell

• Nested functions
• Hard to read & debug
• Poor error handling

Promises solve:

• Inversion of control
• Error propagation
• Readability

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3. What Is a Promise?

Definition

A Promise is an object that represents:

The eventual completion or failure of an asynchronous operation.

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4. Promise States

State	Meaning
Pending	Initial state
Fulfilled	Resolved successfully
Rejected	Failed

A promise is settled once fulfilled or rejected.

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5. Creating Promises

• Executor function
• resolve and reject
• Single settlement rule

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6. Consuming Promises

.then()

• Handles fulfillment
• Chainable

.catch()

• Handles rejection

.finally()

• Cleanup logic

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7. Promise Chaining

• Return values auto-wrapped into promises
• Enables sequential async flow
• Errors propagate downstream

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8. Async / Await Deep Dive

• Syntactic sugar over promises
• Looks synchronous
• Uses microtask queue internally

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9. Error Handling Strategies

• Try/catch with async/await
• Centralized error handlers
• Graceful degradation

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10. Promise Combinators

Promise.all

• Runs in parallel
• Fails fast

Promise.allSettled

• Waits for all results
• No short-circuit

Promise.race

• First settled wins

Promise.any

• First fulfilled wins

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11. Parallel vs Sequential Execution

Pattern	Use Case
Sequential	Dependent calls
Parallel	Independent APIs

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12. API Interaction Basics

What Is an API?

• Application Programming Interface
• Communication contract between systems

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13. HTTP Fundamentals

• Methods: GET, POST, PUT, PATCH, DELETE
• Status codes:
  • 2xx Success
  • 4xx Client errors
  • 5xx Server errors

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14. Fetching Data

Fetch API

• Promise-based
• Returns Response object
• Requires manual JSON parsing

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15. Request & Response Lifecycle

1. Client request
2. DNS lookup
3. TCP handshake
4. Server processing
5. Response parsing

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16. Headers & Payloads

• Content-Type
• Authorization
• JSON vs FormData

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17. Authentication in APIs

• API keys
• Tokens (JWT)
• OAuth (intro)

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18. Handling API Errors

• Network errors
• Timeout
• Non-200 responses
• JSON parsing failures

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19. Retry & Timeout Patterns

• Exponential backoff
• Circuit breakers
• AbortController

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20. Caching API Responses

• Browser cache
• In-memory cache
• CDN caching
• Stale-while-revalidate

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21. Security Best Practices

• HTTPS only
• Never expose secrets
• CORS understanding
• Input validation

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22. Rate Limiting & Throttling

• Prevent abuse
• Improve stability
• Client-side vs server-side

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23. Pagination & Infinite Scroll

• Offset-based
• Cursor-based
• Performance considerations