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Lettable operators in RxJs

After upgrading to Angular 5 (and having some trouble with RxJs but that is for another post), I noticed the introduction of "lettable operators", which can be accessed in rxjs/operators.

What is a lettable operator?

A lettable operator is basically any function that returns a function with the signature: <T, R>(source: Observable<T>) => Observable<R>. Euhm, what?!

Simply put, operators (like filter, map, …) are no longer tied to an Observable directly but can be used with the current let operator(explaining the name). This means you can no longer use the dot-chaining, but will have to use another way to compose your operators.

Therefore is a pipe method built into Observable now at Observable.prototype.pipe:

Why lettable operators?

Lettable operators were introduced to solve the following problems with the dot-chaining(from the documentation):

  1. Any library that imports a patch operator will augment the Observable.prototype for all consumers of that library, creating blind dependencies. If the library removes their usage, they unknowingly break everyone else. With lettables, you have to import the operators you need into each file you use them in.
  2. Operators patched directly onto the prototype are not "tree-shakeable" by tools like rollup or webpack. Lettable operators will be as they are just functions pulled in from modules directly.
  3. Unused operators that are being imported in apps cannot be detected reliably by any sort of build tooling or lint rule. That means that you might import scan, but stop using it, and it's still being added to your output bundle. With lettable operators, if you're not using it, a lint rule can pick it up for you.
  4. Functional composition is awesome. Building your own custom operators becomes much, much easier, and now they work and look just like all other operators from rxjs. You don't need to extend Observable or override lift anymore.

In short lettable operators will improve tree shaking and make it easier to create custom operators.

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