Promise vs. Observable - Angular 5 and 4

Promise vs. Observable - Angular 5 and 4

What is the difference between observables and promises on Angular 5?
What Is Promise?
Promise handles a single event when an asynchronous (async) operation completes or fails.
It helps us to run functions asynchronously and return only one value (or exception) when executed.
Promise is not Lazy.

What Is Observable?
Observable is like a Stream and allows us to pass zero or more events where the callback is called for each events.

Observable provides operators like map, forEach, reduce and similar to an array.
Observable is preferred over Promise because it provides the features of Promise and more.

Promise -
1.      Returns a single value
2.      Not cancellable - It can be Reject or Resolve
3.      More readable code with try/catch and async/await
4.      Promise is not Lazy.

Observable -
1.      Works with multiple values over time
2.      Cancellable
3.      By default, it is Lazy as it emits values when time progresses.
4.      It is cancel-able/retry-able and supports operators such as map, filter, reduce etc.
5.      Use Reactive Extensions (RxJS)
6.      An array whose items arrive asynchronously over time

Both Promises and Observables will help us work with the asynchronous functionalities in JavaScript applications.

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