What is promise?

A promise is a special JavaScript object that represents a value that may be available now, later, or never. It lets you write code that works with asynchronous operations (like loading data from a server) in a way that looks more like normal, step‑by‑step code.

Let's break it down

A promise has three possible states:

  • Pending - the operation hasn’t finished yet.
  • Fulfilled - the operation completed successfully and produced a result.
  • Rejected - the operation failed and gave an error. You attach callbacks to a promise using .then() for success and .catch() for errors. The callbacks run only after the promise leaves the pending state.

Why does it matter?

Without promises, you’d have to nest many callbacks (the “callback hell”) which quickly becomes hard to read and maintain. Promises give you a clear, linear way to handle async work, making code easier to understand, test, and debug.

Where is it used?

  • Fetching data from APIs with fetch() or libraries like Axios.
  • Reading files in Node.js with fs.promises.
  • Any modern browser or Node.js API that performs I/O, timers, or animation.
  • Inside async/await functions, which are just syntactic sugar over promises.

Good things about it

  • Chainable: You can link multiple async steps together with .then().
  • Error handling: A single .catch() can handle errors from any step in the chain.
  • Composability: Combine several promises with Promise.all, Promise.race, etc.
  • Standardized: Built into the language, so you don’t need extra libraries for basic async work.

Not-so-good things

  • Learning curve: Beginners may find the three states and chaining confusing at first.
  • Debugging: Stack traces can be less clear because the code runs in separate ticks of the event loop.
  • Over‑use: Wrapping already‑synchronous code in a promise adds unnecessary complexity.
  • Potential for unhandled rejections: Forgetting a .catch() can cause silent failures in some environments.