Friday 16 February 2024

What is the difference between Promise and Observable Angular? 049

 What is the difference between Promise and Observable Angular?

In Angular, both Promises and Observables are mechanisms for handling asynchronous operations, but they offer distinct approaches and characteristics. Here's a breakdown of their key differences:


  • Definition: Represent the eventual completion (with a value) or failure of an asynchronous operation.

  • Resolution: Resolve with a single value when the operation is successful, or reject with an error value if it fails.

  • Chaining: You can chain .then() and .catch() methods to handle successive operations based on the resolved value or error.

  • Use Cases:

  • Suitable for fetching a single piece of data or performing a one-time asynchronous task.

  • Ideal for simple data fetching or scenarios where a single result is expected.


  • Definition: Represent streams of data emitted over time by an asynchronous source.

  • Emission: Can emit multiple values over time, including initial data, intermediate updates, and potentially an error.

  • Subscription: Requires subscribing to an Observable to receive its emitted values.

  • Use Cases:

  • Well-suited for handling data streams, real-time updates, or situations where multiple values are expected.

  • Useful for long-running operations, user actions that trigger continuous data flow, or communication with servers using WebSockets.

Key Differences:




Value Type

Single value upon resolution/rejection

Stream of values over time


Resolves/rejects once

Emits data indefinitely or errors out


.then() and .catch() for subsequent actions

subscribe() with callback functions

Use Cases

One-time asynchronous tasks

Data streams, real-time updates

When to Choose Which:

  • Use Promises for fetching a single piece of data, simple asynchronous operations, or when you only need the final result.

  • Use Observables for handling data streams, real-time updates, or situations where multiple values are expected over time.

Here's an analogy:

  • Imagine ordering food online. A Promise is like waiting for your entire order to be delivered at once. An Observable is like receiving individual items from your order as they become ready.

By understanding the strengths and use cases of Promises and Observables, you can make informed decisions when handling asynchronous operations in your Angular applications, ensuring optimal performance and appropriate data handling strategies.

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