Merge pull request #19 from dphilipson/feature/is-type-guard

Add isType type guard function

Author: dphilipson

README.md

@@ -85,8 +85,8 @@ export type Status = Enum<typeof Status>;
 console.log(Status.RUNNING); // -> "running"
 ```
 
-Two helper functions are provided: `Enum.keys()` and `Enum.values()`, which resemble `Object.keys()`
-and `Object.values()` but provide strict typing in their return type:
+Several helper functions are provided. First are `Enum.keys()` and `Enum.values()`, which resemble
+`Object.keys()` and `Object.values()` but provide strict typing in their return type:
 
 ``` javascript
 const FileType = Enum({
@@ -105,6 +105,27 @@ const values = Enum.values(FileType);
 // Return value: ["application/pdf", "text/plain", "image/jpeg"] (not necessarily in that order)
 ```
 
+Also available is `Enum.isType()`, which checks if a value is of a given enum type and can be used
+as a type guard.
+
+``` javascript
+const Color = Enum({
+    BLACK: "black",
+    WHITE: "white",
+});
+type Color = Enum<typeof Color>;
+
+let selectedColor: Color;
+
+const colorString = getUserInputString(); // Unsanitized string.
+if (Enum.isType(Color, colorString)) {
+    // Allowed because within type guard.
+    selectedColor = colorString;
+} else {
+    throw new Error(`${colorString} is not a valid color`);
+}
+```
+
 ## Motivation
 
 Enums are useful for cleanly specifying a type that can take one of a few specific values.

__tests__/test.ts

@@ -25,7 +25,7 @@ describe("Enum.keys", () => {
         });
         expect(Enum.keys(e)).toEqual(expect.arrayContaining([ "WHITE", "BLACK" ]));
     });
-};
+});
 
 describe("Enum.values", () => {
     it("returns the values of an enum object", () => {
@@ -36,3 +36,18 @@ describe("Enum.values", () => {
         expect(Enum.values(e)).toEqual(expect.arrayContaining([ "white", "black" ]));
     });
 });
+
+describe("Enum.isType", () => {
+    const Color = Enum({
+        BLACK: "black",
+        WHITE: "white",
+    });
+
+    it("returns true if value is of type", () => {
+        expect(Enum.isType(Color, "black")).toBe(true);
+    });
+
+    it("returns false if value is not of type", () => {
+        expect(Enum.isType(Color, "BLACK")).toBe(false);
+    });
+});

package.json

@@ -33,7 +33,7 @@
     "watch": "yarn run clean && tsc --watch"
   },
   "devDependencies": {
-    "@types/jest": "^16.0.4",
+    "@types/jest": "^19.2.2",
     "jest": "^18.1.0",
     "ts-jest": "^18.0.1",
     "tslint": "^4.3.1",

src/index.ts

@@ -33,4 +33,10 @@ export namespace Enum {
         }
         return result;
     }
+
+    export function isType<
+        T extends { [_: string]: any }
+    >(e: T, value: string): value is Enum<T> {
+        return values(e).indexOf(value) !== -1;
+    }
 }

yarn.lock

@@ -2,9 +2,9 @@
 # yarn lockfile v1
 
 
-"@types/jest@^16.0.4":
-  version "16.0.4"
-  resolved "https://registry.yarnpkg.com/@types/jest/-/jest-16.0.4.tgz#31bdd13e2bdfa71498b022494ba04d407f9f8230"
+"@types/jest@^19.2.2":
+  version "19.2.2"
+  resolved "https://registry.yarnpkg.com/@types/jest/-/jest-19.2.2.tgz#71f428be2fa6eb9f15bb0abc3cade67905f94839"
 
 abab@^1.0.0:
   version "1.0.3"