Add TypeScript definitions to enforce or infer whether properties are required.

jest.config.js

@@ -0,0 +1,8 @@
+module.exports = {
+  transform: {
+    '^.+\\.js$': 'babel-jest',
+    '^.+\\.tsx?$': 'ts-jest'
+  },
+  testRegex: '(/__tests__/.*|(\\.|/)(test|spec))\\.[jt]sx?$',
+  moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx', 'json', 'node']
+};

package.json

@@ -33,18 +33,21 @@
     "@babel/cli": "^7.2.3",
     "@babel/core": "^7.3.4",
     "@babel/preset-env": "^7.3.4",
+    "@types/jest": "^24.0.11",
+    "@typescript-eslint/eslint-plugin": "^1.5.0",
     "babel-jest": "^24.5.0",
     "babel-plugin-lodash": "^3.3.4",
+    "eslint": "^5.0.0",
     "eslint-config-jest-files": "^0.1.3",
     "eslint-config-typescript-basic": "^1.0.1",
     "eslint-config-unicorn-camelcase": "^0.1.1",
     "eslint-plugin-prettier": "^3.0.1",
-    "eslint-plugin-typescript": "^0.14.0",
     "husky": "^0.14.3",
     "jest": "^24.5.0",
     "prettier": "^1.16.4",
     "pretty-quick": "^1.10.0",
     "rimraf": "^2.6.3",
+    "ts-jest": "^24.0.1",
     "typescript": "^3.3.3333",
     "typescript-eslint-parser": "^22.0.0",
     "xo": "^0.24.0"
@@ -67,7 +70,8 @@
   "prettier": {
     "singleQuote": true,
     "bracketSpacing": true,
-    "trailingComma": "none"
+    "trailingComma": "none",
+    "endOfLine": "auto"
   },
   "jest": {
     "collectCoverage": true,

src/index.d.ts

@@ -1,9 +1,16 @@
-export interface FluxStandardAction<Payload, Meta = undefined> {
+/**
+ * A Flux Standard action with optional payload and metadata properties.
+ */
+export interface FluxStandardAction<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> {
   /**
    * The `type` of an action identifies to the consumer the nature of the action that has occurred.
    * Two actions with the same `type` MUST be strictly equivalent (using `===`)
    */
-  type: string;
+  type: Type;
   /**
    * The optional `payload` property MAY be any type of value.
    * It represents the payload of the action.
@@ -26,36 +33,222 @@ export interface FluxStandardAction<Payload, Meta = undefined> {
   meta?: Meta;
 }
 
+/**
+ * An extension of the Flux Standard action that represents an action containing an error as its payload.
+ */
 export interface ErrorFluxStandardAction<
-  CustomError extends Error,
-  Meta = undefined
-> extends FluxStandardAction<CustomError, Meta> {
+  CustomError extends Error = Error,
+  Meta = undefined,
+  Type extends string = string
+> extends FluxStandardAction<CustomError, Meta, Type> {
+  /**
+   * The required `error` property MUST be set to `true` if the action represents an error.
+   */
   error: true;
 }
 
 /**
  * Alias for FluxStandardAction.
  */
-export type FSA<Payload, Meta = undefined> = FluxStandardAction<Payload, Meta>;
+export type FSA<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardAction<Payload, Meta, Type>;
 
 /**
  * Alias for ErrorFluxStandardAction.
  */
 export type ErrorFSA<
-  CustomError extends Error,
-  Meta = undefined
-> = ErrorFluxStandardAction<CustomError, Meta>;
+  CustomError extends Error = Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardAction<CustomError, Meta, Type>;
 
 /**
  * Returns `true` if `action` is FSA compliant.
  */
-export function isFSA<Payload, Meta = undefined>(
-  action: any
-): action is FluxStandardAction<Payload, Meta>;
+export function isFSA<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+>(action: any): action is FluxStandardAction<Payload, Meta, Type>;
 
 /**
  * Returns `true` if `action` is FSA compliant error.
  */
-export function isError<CustomError extends Error, Meta = undefined>(
-  action: any
-): action is ErrorFluxStandardAction<CustomError, Meta>;
+export function isError<
+  CustomError extends Error = Error,
+  Meta = undefined,
+  Type extends string = string
+>(action: any): action is ErrorFluxStandardAction<CustomError, Meta, Type>;
+
+/**
+ * A Flux Standard action with a required payload property.
+ */
+export interface FluxStandardActionWithPayload<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> extends FluxStandardAction<Payload, Meta, Type> {
+  /**
+   * The required `payload` property MAY be any type of value.
+   * It represents the payload of the action.
+   * Any information about the action that is not the type or status of the action should be part of the `payload` field.
+   * By convention, if `error` is `true`, the `payload` SHOULD be an error object.
+   * This is akin to rejecting a promise with an error object.
+   */
+  payload: Payload;
+}
+/**
+ * Alias for FSAWithPayload
+ */
+export type FSAWithPayload<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardActionWithPayload<Payload, Meta, Type>;
+
+/**
+ * A Flux Standard action with a required metadata property.
+ */
+export interface FluxStandardActionWithMeta<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> extends FluxStandardAction<Payload, Meta, Type> {
+  /**
+   * The required `meta` property MAY be any type of value.
+   * It is intended for any extra information that is not part of the payload.
+   */
+  meta: Meta;
+}
+/**
+ * Alias for FluxStandardActionWithMeta
+ */
+export type FSAWithMeta<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardActionWithMeta<Payload, Meta, Type>;
+
+/**
+ * A Flux Standard action with required payload and metadata properties.
+ */
+export type FluxStandardActionWithPayloadAndMeta<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardActionWithPayload<Payload, Meta, Type> &
+  FluxStandardActionWithMeta<Payload, Meta, Type>;
+/**
+ * Alias for FluxStandardActionWithPayloadAndMeta
+ */
+export type FSAWithPayloadAndMeta<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardActionWithPayloadAndMeta<Payload, Meta, Type>;
+
+/**
+ * A Flux Standard action with inferred requirements for the payload and metadata properties.
+ * The `payload` and `meta` properties will be required if the corresponding type argument
+ * if not the `undefined` type.
+ */
+export type FluxStandardActionAuto<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = Payload extends undefined
+  ? (Meta extends undefined
+      ? FluxStandardAction<Payload, Meta, Type>
+      : FluxStandardActionWithMeta<Payload, Meta, Type>)
+  : (Meta extends undefined
+      ? FluxStandardActionWithPayload<Payload, Meta, Type>
+      : FluxStandardActionWithPayloadAndMeta<Payload, Meta, Type>);
+/**
+ * Alias for FluxStandardActionAuto
+ */
+export type FSAAuto<
+  Payload = undefined,
+  Meta = undefined,
+  Type extends string = string
+> = FluxStandardActionAuto<Payload, Meta, Type>;
+
+/**
+ * A Flux Standard Error Action with a required payload property.
+ */
+export type ErrorFluxStandardActionWithPayload<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardAction<CustomError, Meta, Type> &
+  FluxStandardActionWithPayload<CustomError, Meta, Type>;
+/**
+ * Alias for ErrorFluxStandardActionWithPayload
+ */
+export type ErrorFSAWithPayload<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardActionWithPayload<CustomError, Meta, Type>;
+
+/**
+ * A Flux Standard Error Action with a required metadata property.
+ */
+export type ErrorFluxStandardActionWithMeta<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardAction<CustomError, Meta, Type> &
+  FluxStandardActionWithMeta<CustomError, Meta, Type>;
+/**
+ * Alias for ErrorFluxStandardActionWithMeta
+ */
+export type ErrorFSAWithMeta<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardActionWithMeta<CustomError, Meta, Type>;
+
+/**
+ * A Flux Standard Error Action with required payload and metadata properties.
+ */
+export type ErrorFluxStandardActionWithPayloadAndMeta<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardActionWithPayload<CustomError, Meta, Type> &
+  ErrorFluxStandardActionWithMeta<CustomError, Meta, Type>;
+/**
+ * Alias for ErrorFluxStandardActionWithPayloadAndMeta
+ */
+export type ErrorFSAWithPayloadAndMeta<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardActionWithPayloadAndMeta<CustomError, Meta, Type>;
+
+/**
+ * A Flux Standard Error action with inferred requirements for the payload and metadata properties.
+ * The `payload` and `meta` properties will be required if the corresponding type argument
+ * if not the `undefined` type.
+ *
+ * Note: The `payload` property will always be required, since the `CustomError` type argument does
+ * not allow for specification of the `undefined` type.
+ */
+export type ErrorFluxStandardActionAuto<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = Meta extends undefined
+  ? ErrorFluxStandardActionWithPayload<CustomError, Meta, Type>
+  : ErrorFluxStandardActionWithPayloadAndMeta<CustomError, Meta, Type>;
+/**
+ * Alias for ErrorFluxStandardActionAuto
+ */
+export type ErrorFSAAuto<
+  CustomError extends Error,
+  Meta = undefined,
+  Type extends string = string
+> = ErrorFluxStandardActionAuto<CustomError, Meta, Type>;

test/fsaAuto.test.ts

@@ -0,0 +1,16 @@
+import { FSAAuto, isFSA } from '../src';
+
+describe('Usage of FSAAuto (automatically infer required properties', () => {
+  it('must specify payload property even when using a union with undefined', () => {
+    const fsa_with_payload = { type: 'TEST', payload: undefined };
+    expectOptionalPayload(fsa_with_payload);
+
+    const fsa_without_payload = { type: 'TEST' };
+    // Not possible to cast!!!
+    // expectOptionalPayload(fsa_without_payload);
+
+    function expectOptionalPayload(fsa: FSAAuto<string | undefined>) {
+      expect(fsa.payload).toBeUndefined();
+    }
+  });
+});

test/typeFSA.test.ts

@@ -0,0 +1,23 @@
+import { FSA } from '../src';
+
+const ACTION_TYPE_1 = 'ACTION_TYPE_1';
+type ACTION_TYPE_1 = typeof ACTION_TYPE_1;
+type FSA_ACTION_TYPE_1 = FSA<undefined, undefined, ACTION_TYPE_1>;
+
+const assertNever = (x: never): never => {
+  throw new Error(`Unexpected value: ${x}.`);
+};
+
+const assertTypeValue = (fsa: FSA_ACTION_TYPE_1) => {
+  expect(fsa.type).toBe(ACTION_TYPE_1);
+};
+
+describe('FluxStandardAction<Payload, Meta, Type>', () => {
+  it('enables TypeScript action type enforcement', () => {
+    const fsa_strict: FSA_ACTION_TYPE_1 = { type: ACTION_TYPE_1 };
+    assertTypeValue(fsa_strict);
+    if (fsa_strict.type !== ACTION_TYPE_1) {
+      throw assertNever(fsa_strict.type);
+    }
+  });
+});

tsconfig.json

@@ -4,7 +4,5 @@
     "strictNullChecks": true,
     "target": "es5"
   },
-  "files": [
-    "test/typings.test.ts"
-  ]
+  "files": ["src/index.d.ts"]
 }