rxjs-tslint-rules

What is it?

rxjs-tslint-rules is small set of TSLint rules to help manage projects that use rxjs/add/... imports and to highlight other potential problems.

Why might you need it?

When using imports that patch Observable:

import { Observable } from "rxjs/Observable";
import from "rxjs/add/observable/of";
import from "rxjs/add/operator/map";

TypeScript will see the merged declarations in all modules, making it difficult to find rxjs/add/... imports that are missing from modules in which patched observables and operators are used.

This can cause problems, as whether or not Observable is patched then depends upon the order in which the modules are executed.

The rules in this package can be used to highlight missing - or unused - imports and other potential problems with RxJS.

Install

Install the package using NPM:

npm install rxjs-tslint-rules --save-dev

Update your tslint.json file to extend this package:

{
  "extends": [
    "rxjs-tslint-rules"
  ],
  "rules": {
    "rxjs-add": { "severity": "error" },
    "rxjs-no-unused-add": { "severity": "error" }
  }
}

Rules

The package includes the following rules:

Options

The rxjs-add rule takes an optional object with the property file. This is the path of the module - relative to the tsconfig.json - that imports the patched observables and operators.

For example:

"rules": {
  "rxjs-add": {
    "options": [{
      "allowElsewhere": false,
      "allowUnused": false,
      "file": "./source/rxjs-imports.ts"
    }],
    "severity": "error"
  }
}

Specifying the file option allows all of the patched observables and operators to be kept in a central location. Said module should be imported before other modules that use patched observables and operators. The importation of said module is not enforced; the rule only ensures that it imports observables and operators that are used in other modules.

If file is specified, the allowElsewhere and allowUnused options can be used to configure whether or not patched imports are allowed in other files and whether or not unused patched imports are allowed. Both allowElsewhere and allowUnused default to false.

Note that there is no file option for the rxjs-no-unused-add rule, so that rule should not be used in conjunction with the rxjs-add rule - if the file option is specified for the latter. Use the rxjs-add rule's allowUnused option instead.

If the file option is not specified, patched observables and operators must be imported in the modules in which they are used.

Gotchas

@angular/cli

Angular's CLI runs TSLint three times:

• first, with application files from src/ (using src/tsconfig.app.json);
• then with the test files from src/ (using src/tsconfig.spec.json);
• and, finally, with files from e2e/ (using e2e/tsconfig.e2e.json).

If you are using the file option of the rxjs-add rule to ensure patched observables and operators are kept in a central location, there are some configuration changes that you should make:

• I'd recommend switching off rxjs-add for the e2e linting, as the central file isn't necessary or appropriate. The simplest way to do this is to create an e2e/tslint.json file with the following content:

    {
      "extends": ["../tslint.json"],
      "rules": {
        "rxjs-add": { "severity": "off" }
      }
    }

• And, for the test linting, I'd recommend adding the central file to the TypeScript configuration. If the central file is, say, src/rxjs.imports.ts, add that file to the "files" in src/tsconfig.spec.json:

    "files": [
      "rxjs.imports.ts",
      "test.ts"
    ]

With these changes, the rule should play nice with the CLI's running of TSLint. If you are using "allowUnused": false and receive errors about unused operators, you should make sure that files in which those operators are used are imported into at least one test. (The rule will walk all files included in the TypeScript program - not just the specs - so if an unused error is effected, the file using the unused operator is not present in the program and needs to be imported into a test.)

Observable.create

Observable.create is declared as a Function, which means that its return type is any. This results in an observable that's not seen by the rules, as they use TypeScript's TypeChecker to determine whether or not a call involves an observable.

The rule implementations include no special handling for this case, so if spurious errors are effected due to Observable.create, explicit typing can resolve them. For example:

const ob: Observable<number> = Observable.create((observer: Observer<number>) => { ...

More

There are some examples of policies that can be implemented using particular rule combinations in: Managing RxJS Imports with TSLint; and Understanding Lettable Operators.

Christian Liebel has written about his approach to importing RxJS in his blog post: Angular & TypeScript: How to Import RxJS Correctly?