I want to understand the exact differences between these functions. Since both are used for asynchronous programming, where should we use promises, and where should we use subscriptions?
In asynchronous programming, promises and subscriptions (often via Observables) are two common approaches to handle asynchronous operations, each with its own use cases and advantages.
Promises
A promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value. A promise can only handle a single event. Here’s when to use promises:
Single Asynchronous Operations: Use promises when you only need to handle a single async operation that will either succeed or fail, like fetching data from an API.
Simpler Syntax: Promises provide a simpler syntax with then(), catch(), and finally() methods, which can make your code easier to read and manage when dealing with a single asynchronous response or error.
Chaining Asynchronous Operations: Promises are well-suited for scenarios where you need to perform several asynchronous tasks in a sequence, where each subsequent task starts only after the previous one has completed.
Subscriptions (Observables)
Subscriptions often involve Observables, which can handle multiple values over time, from zero to an infinite number of values. Use subscriptions in the following scenarios:
Multiple Asynchronous Events: Subscriptions are ideal for dealing with streams of events or data, such as user input, progress indicators, or other recurring events.
More Control Over the Asynchronous Streams: Observables provide operators like map, filter, debounceTime, etc., which allow for more control and flexibility in processing streams of data.
Non-blocking Operations: In cases where your application needs to continue reacting to incoming events without waiting for previous operations to complete, subscriptions allow more granular control over these streams.
Key Differences
Completion: Promises are either resolved or rejected a single time, whereas Observables can emit multiple values over time and must be manually unsubscribed to avoid memory leaks and unwanted behavior.
Use of Operators: Observables can be transformed, combined, and manipulated using a variety of operators provided by libraries like RxJS, which can be extremely powerful for handling complex asynchronous logic.
Error Handling: Promises handle errors in a linear path with catch(), while Observables handle errors in the stream flow, which can be caught and managed using operators like catchError.