What is the difference between an observable and a promise? A comprehensive guide.

Have you ever heard of observables and promises? If you’re new to the world of programming, terms like these might seem a bit daunting or confusing. But don’t worry, in this article, we’re going to break down the differences between observables and promises and make it easy for you to understand.

In short, both observables and promises are methods that are used in asynchronous programming. However, while they have similarities, they also have distinct differences that make them useful in different situations. Observables are more versatile and can do things promises can’t, while promises are simpler and more straightforward to use.

If you’re a developer and you’re trying to choose between the two, it’s important to understand these differences so you can use the right method for your needs. Whether you’re working on front-end web development or back-end systems, knowing how to use observables and promises effectively can make a big difference in your coding productivity and the success of your projects. So, let’s get started breaking it all down!

Understanding Observables

Observables and Promises in JavaScript are both used to handle asynchronous operations. However, they are different in some distinct ways. Observables can return multiple values over time, while a Promise can only resolve or reject once with one value. Observables can be canceled, whereas Promises cannot. Observables can also be transformed using operators.

  • Multiple Values: As mentioned, Observables can return multiple values over a period of time, while Promises only return a single value. For example, an Observable could be used to display the progress of file uploads or the changes in a user’s location over time.
  • Cancellation: Observables can be canceled, which means that the subscription is terminated and no further values are emitted. Promises are not able to be canceled once they have been resolved or rejected.
  • Operators: One significant advantage of Observables is the ability to transform them using operators. These operators can be used to filter, map, or reduce the values emitted by an Observable. This provides a great degree of flexibility in how Observable data is managed and manipulated.

Identifying Promises

When working with asynchronous code in JavaScript, it’s common to encounter both observables and promises. While these two concepts are related, they have some important differences that are worth understanding. In this article, we’ll take a look at how to identify promises in your code, and what sets them apart from observables.

  • Method return types: One way to identify promises in your code is by looking at function return types. Promises can be created with the Promise.resolve() and Promise.reject() methods, and functions that use these methods typically return a promise object.
  • .then() and .catch() methods: Promises often require the use of a .then() method, which is used to specify the code that should be executed when the promise resolves successfully. Additionally, many promises also have a .catch() method that can be used to handle errors if they occur during the promise’s execution.
  • Callbacks: Another way to identify promises is by looking for the use of callbacks in your code. Promises often use callbacks, either to handle successful resolution of the promise, or to handle errors that occur during its execution.

While observables and promises might seem similar at first glance, there are several key differences between these two concepts that can make one more suitable than the other depending on the situation.

Next, we’ll take a look at some of the differences between observables and promises, and how you can decide which one is the best choice for your particular use case.

Observables vs Promises: The Conceptual Difference

When it comes to asynchronous programming, Observables and Promises are two commonly used constructs. While they both aim to handle asynchronous operations, they differ in their approach and functionalities. Let’s explore the conceptual differences between the two.

The Role of Time

  • Promises are meant to handle single future values.
  • Observables can handle multiple future values or even an infinite stream of future values over time.
  • Promises resolve once and emit a single value or rejection.
  • Observables can emit zero, one, or many values over a period of time and can be cancelled or unsubscribed from.

These differences in handling values over time are what distinguish Observables from Promises. Asynchronous operations can produce values from time to time, making Observables a better construct to use for continuous feedback on data across time.

Their Usage

Promises are typically used for short-lived asynchronous operations like fetching data or making API calls. Once the response is received, a Promise resolves to a single value and completes, which makes them ideal for operations that happen only once and do not need to be repeated.

On the other hand, Observables have potential value when it comes to continuous or longer-term asynchronous operations. They are often used for listening to changes on user interface elements or event-driven functions where values keep changing over time. Observables are beneficial in use cases that span beyond fetching data or making API calls, handling ongoing and diverse changes in data.

The Overhead

Promises are comparatively simple and lightweight tools. They are easier to understand and use, making them great for simple asynchronous operations.

Observables, on the other hand, have a higher learning curve. They have additional overhead due to their ability to deal with multiple values and changes over time, and their complexity in dealing with multithreading. Observables are also relatively challenging to debug, as their complex structure requires considerable attention to specific code sections and flow control.

Nevertheless, the overhead cost of learning and using Observables can be worth it as they provide a more extensive range of functionality.

Promises Observables
Simple and lightweight Complex and heavyweight
Single-value handling Multiple-value handling
Easy to debug Relatively hard to debug

In conclusion, while Observables and Promises perform a similar function in handling asynchronous operations, they differ in significant ways. Observables are better suited for continuous and long-term operations, while Promises are ideal for short-term and one-time operations. Observables are more complex, whereas Promises are relatively simple and easier to use. Hopefully, this article has given you a starting point to understand the fundamental differences between these two constructs.

Comparison of Lazy and Eager Execution

When it comes to observables and promises, one of the major differences is in their execution behavior. Observables can be either “lazy” or “eager” depending on when they are subscribed to, while promises are always “eager”.

  • Lazy Execution: With lazy execution, the observable does not start emitting data until there is a subscriber. This means that the code in the observable is not executed until someone is actively listening for new data. This is useful when dealing with large amounts of data or resources that should only be used when needed.
  • Eager Execution: With eager execution, code in an observable is executed regardless of whether there is an active subscriber or not. This means that the data starts emitting immediately once the observable is created. Eager execution is useful for quickly getting data or resources that are needed immediately.

On the other hand, promises always execute eagerly. When a promise is created, the code within it is immediately executed, and the result is immediately available. This means that promises may return data that is not immediately needed, which can lead to wasted resources.

Here is a comparison between the execution behaviors of observables and promises:

Observable Promise
Execution Behavior Lazy or Eager Eager
Multiple Values Yes No
Error Handling Can emit multiple error events Only one error event
Cancellation Yes No

Overall, understanding the differences between lazy and eager execution is important for developers working with observables and promises, as it can help them optimize their code and make more efficient use of resources.

Handling Multiple Values in Observables and Promises

When it comes to handling multiple values in Observables and Promises, there are significant differences in how they function.

The primary difference lies in how they handle and deliver data. Observables are capable of delivering multiple pieces of data over time, whereas Promises can only deliver a single value upon completion.

  • Observables: As mentioned earlier, Observables can deliver multiple values over time, making them ideal for scenarios where we need to handle multiple streams of data or update our application with new pieces of information frequently. Observables are not just limited to emitting data—they can also handle errors and completion events, which makes them more versatile than Promises. We can use operators like map and filter to transform and manipulate the data emitted by Observables, which further extends their functionality.
  • Promises: On the other hand, Promises are designed to handle a single piece of data, which is returned upon completion. Promises are great for scenarios where we have a one-time operation that returns a value, such as fetching data from a server, calculating a value, etc. Promises can be chained using then() and catch() methods to handle the resolved and rejected outcomes, but they do not have the ability to emit multiple values over time, which makes them less versatile than Observables.

Let’s take a look at an example to understand the differences between the two:

Suppose we have an API endpoint that returns the stock prices of a particular company for the last month as an array of numbers. To fetch this data, we can use either an Observable or a Promise, depending on our requirements.

Observable Promise

import { Observable } from ‘rxjs’;

const stockPrices$ = new Observable((observer) => {
.then((response) => response.json())
.then((data) => {
data.forEach((price) => {
.catch((err) => observer.error(err));

stockPrices$.subscribe((price) => console.log(price));

.then((response) => response.json())
.then((data) => {
console.log(data); // [122.31, 124.65, 126.78, 127.90, …]
.catch((err) => console.error(err));

In the example above, we create an Observable that fetches the data from the API and emits each stock price as it arrives. The Observable’s subscribe() method then logs each price to the console as it is emitted.

On the other hand, the Promise-based solution simply logs the entire array of stock prices to the console once the API request has been completed.

As you can see, when it comes to handling multiple values, Observables provide a much more elegant and flexible solution than Promises. However, if you’re dealing with a one-time operation that returns a single value, Promises can be an excellent choice.

Error Handling in Observables and Promises

Both Observables and Promises provide ways of handling errors in asynchronous code, but they do it in different ways.

One key difference is that Promises use a catch() block to handle errors that occur during the execution of the asynchronous operation, while Observables use an error() method that is called when an error occurs.

Error Handling in Observables and Promises

  • When using Promises, errors are propagated down the chain until a catch() block is reached that can handle the error.
  • With Observables, errors are treated as part of the normal data flow, and the error() method is called instead of the next() method when an error occurs.
  • This means that when using Observables, errors are handled more proactively, and can be caught and handled at any point in the stream.

Error Handling in Observables and Promises

When it comes to error handling, Observables provide more flexibility than Promises. For example, Observables allow you to retry an operation when an error occurs, or to switch to a different Observable if an error occurs. This makes it easier to handle unexpected errors and recover from them.

Another benefit of using Observables is that they provide a way to handle multiple errors at once. The error() method can take an Error object or an array of Error objects as a parameter, which makes it easy to handle multiple errors as a group.

Error Handling in Observables and Promises

It’s important to note that error handling in Observables and Promises can differ depending on the specific library or framework being used. For example, some libraries may provide additional methods or properties for error handling.

Observables Error Handling Promises Error Handling
Observable.error() method catch() block
Retry an operation on error No retry functionality
Handle multiple errors at once Errors must be handled one at a time

Ultimately, the choice between using Observables or Promises for error handling will depend on the specific needs of the application or project. Both approaches have their strengths and weaknesses, and it’s important to carefully consider which one is the best fit for the task at hand.

Advantages and Disadvantages of Using Observables over Promises

If you are a JavaScript developer, you are probably familiar with two asynchronous paradigms that are used when handling events: Observables and Promises. Observables and Promises are both designed to handle asynchronous data flows but do so in different ways.

This article will discuss the differences between Observables and Promises, and the advantages and disadvantages of using Observables over Promises.

Advantages of Using Observables over Promises

  • Observer Pattern: Observables work on the observer pattern. This means that multiple subscribers can listen to a single Observable instance. On the other hand, Promises can only have a single callback function attached to it.
  • Cancellation: Observables can be cancelled at any time. Promises, on the other hand, cannot be cancelled once they are initiated.
  • Lazy Execution: Observables are lazily executed. They won’t run until you call subscribe() on the Observable instance. Promises, on the other hand, are eagerly executed once they are created.
  • Support for Multiple Values: Observables can emit multiple values over time, whereas Promises only resolve once with a single value.

Disadvantages of Using Observables over Promises

Although Observables have many advantages over Promises, there are some disadvantages to using them:

  • Learning Curve: Observables have a steeper learning curve than Promises because the concept of streams and operators is new to many developers.
  • Complexity: Observables can be more complex than Promises due to the number of operators and pipeables that are available.
  • Overkill: If you only need to handle a single asynchronous operation, Promises may be sufficient and using Observables may be overkill.


Both Observables and Promises have their own advantages and disadvantages, and which one you choose to use depends on the complexity of your application and the asynchronous data flows you are handling. However, if you need to handle multiple asynchronous data flows, Observables provide a flexible and powerful way to do so.

Observables Promises
Support for multiple subscribers Only one callback function allowed
Can be cancelled Cannot be cancelled
Lazily executed Eagerly executed
Can emit multiple values Only resolve once with a single value

Overall, it really depends on the situation whether to use Observables or Promises but Observables being a newer technology, it is likely to be supported much more in the future.

What is the difference between an observable and a promise?

1. What is an observable?
An Observable is a stream of data that can be used to handle asynchronous actions. It emits multiple values and can be cancelled.

2. What is a promise?
A Promise is a placeholder that represents a value that is yet to be resolved. It has a single value and resolves only once.

3. What is the difference in how they handle multiple values?
Observables emit multiple values over time while a promise only returns a single value.

4. How do they handle errors?
Observables can handle errors and emit them using the .error() method. A rejected Promise will throw an error and this is the only way to handle errors for promises.

5. How are they executed?
Observables are lazy meaning that they will only execute when subscribed to. Promises execute immediately when created.

Thanks for reading!

We hope this has helped you understand the difference between Observables and Promises. Remember that they serve different purposes with Observables being useful for handling multiple streams of data while Promises are useful for handling a single value that will resolve at a later time. If you have any further questions or comments, feel free to leave them below. Come back soon for more informative articles!

Search Here