rxjs-tslint-rules

What is it?

rxjs-tslint-rules is small set of TSLint rules to:

• help manage projects that use rxjs/add/... imports;
• enforce or disallow Finnish notation; and
• highlight other potential problems (see the rules for details).

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.

There are some examples of policies that can be implemented using particular rule combinations in:

• Managing RxJS Imports with TSLint; and
• Understanding Lettable Operators.

And Christian Liebel has written about his approach to importing RxJS in his blog post:

• Angular & TypeScript: How to Import RxJS Correctly?

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 (none of which are enabled by default):

Options

rxjs-add

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.

rxjs-no-add and rxjs-no-patched

The rxjs-no-add and rxjs-no-patched rules take an optional object with the optional properties allowObservables and allowOperators. The properties can be specified as booleans - to allow or disallow all observables or operators - or as arrays of strings - to allow or disallow a subset of observables or operators.

For example:

"rules": {
  "rxjs-no-patched": {
    "options": [{
      "allowObservables": ["never", "throw"],
      "allowOperators": false
    }],
    "severity": "error"
  }
}

rxjs-no-unsafe-switchmap

The rxjs-no-unsafe-switchmap rule does its best to determine whether or not NgRx effects or redux-observable epics use the switchMap operator with actions for which it could be unsafe.

For example, it would be unsafe to use switchMap in an effect or epic that deletes a resource. If the user were to instigate another delete action whilst one was pending, the pending action would be cancelled and the pending delete might or might not occur. Victor Savkin has mentioned such scenarios in a tweet.

The rule takes an optional object with optional allow and disallow properties. Either can be specified, but not both. The properties can be specifed as regular expression strings or as arrays containing the string fragments that are allowed or disallowed.

If the allow option is specified, any actions that do not match the regular expression or do not contain one the specified fragments will effect an error if switchMap is used.

If the disallow option is specified, any actions that do match the regular expression or do contain one the specified fragments will effect an error if switchMap is used.

If no options are specifed, the rule will default to a set of fragments are are likely to be present in any actions for which switchMap is unsafe.

The following options are equivalent to the rule's default configuration:

"rules": {
  "rxjs-no-unsafe-switchmap": {
    "options": [{
      "disallow": ["add", "create", "delete", "post", "put", "remove", "set", "update"]
    }],
    "severity": "error"
  }
}

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"
    ]

  Alternatively, you can import rxjs.imports.ts directly into tests.ts, like this:

    import "./rxjs.imports";

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.)

If you experience difficulties in configuring the rules with an @angular/cli-generated application, there is an example in this repo of a working configuration. To see the configuration changes that were made to a vanilla CLI application, have a look at this commit.

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>) => { ...