What is the difference between Promises and Observables?
Angular2 Observables, Http, and Separating Services and Components
Observables are compared to other techniques
Observable and Promise:
Observables are declarative; computation does not start until subscription. Promises execute immediately on creation. This makes observables useful for defining recipes that can be run whenever you need the result.
Observable is declarative, and calculations are not triggered until Subscription. This is very different from promises, which are created and implemented immediately.
Observables provide many values. Promises provide one. This makes observables useful for getting multiple values over time.
Observable provides multiple values, whereas Promises provides a single value.
Observables differentiate between chaining and subscription. Promises only have .then() clauses. This makes observables useful for creating complex transformation recipes to be used by other part of the system, without causing the work to be executed.
An Observable can combine complex Transformation recipes with RXJS’s rich Operators for easy in-app reuse, whereas a promise has only one THEN operation.
Observables subscribe() is responsible for handling errors. Promises push errors to the child promises. This makes observables useful for centralized and predictable error handling.