Add new decorator for casting to Boolean

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - stable
+  - 10
   - 8
 
 after_success:

README.md

@@ -245,6 +245,33 @@ import {deserializeArray} from "class-transformer";
 let photos = deserializeArray(Photo, photos);
 ```
 
+## Type casting
+
+By default typescript emits types metadata and we can infer how to cast them. But `Boolean` works not as expected 
+
+
+```typescript
+Boolean('false') === true
+Boolean(undefined) === false
+```
+So there is an opportunity to cast boolean as expected:
+
+```typescript
+class Dto {
+    @ToBoolean()
+    hasFlag: boolean;
+}
+
+plainToClass(Dto, {hasFlag: 'false'})
+// { hasFlag: false }
+plainToClass(Dto, {hasFlag: 0})
+// { hasFlag: false }
+plainToClass(Dto, {hasFlag: 1})
+// { hasFlag: true }
+```
+
+
+
 ## Enforcing type-safe instance
 
 The default behaviour of the `plainToClass` method is to set *all* properties from the plain object,

src/decorators.ts

@@ -18,6 +18,24 @@ export function Transform(transformFn: (value: any, obj: any, transformationType
     };
 }
 
+
+/**
+ * Cast value to boolean type
+ */
+export function ToBoolean() {
+    return Transform((value) => {
+        if (value === "true" || value === "1" || value === 1) {
+            return true;
+        }
+
+        if (value === "false" || value === 0 || value === "0") {
+            return false;
+        }
+
+        return value;
+    });
+}
+
 /**
  * Specifies a type of the property.
  * The given TypeFunction can return a constructor. A discriminator can be given in the options.

test/functional/to-boolean-decorator.spec.ts

@@ -0,0 +1,58 @@
+import {plainToClass, ToBoolean} from "../../src";
+import { expect } from "chai";
+
+describe("Cast to boolean", () => {
+    class Dto {
+        @ToBoolean()
+        hasFlag: boolean;
+    }
+
+   it("Should cast false value as false", () => {
+
+       const result = plainToClass(Dto, { hasFlag: "false" });
+       expect(result.hasFlag).to.be.false;
+   });
+
+    it("Should cast true value as true", () => {
+        const result = plainToClass(Dto, { hasFlag: "true" });
+        expect(result.hasFlag).to.be.true;
+    });
+
+    it("Should not cast undefined as bool", () => {
+        const result = plainToClass(Dto, { hasFlag: undefined });
+        expect(result.hasFlag).to.not.be.equals(true);
+        expect(result.hasFlag).to.not.be.equals(false);
+    });
+
+    it("Should not cast null as bool", () => {
+        const result = plainToClass(Dto, { hasFlag: null });
+        expect(result.hasFlag).to.not.be.equals(true);
+        expect(result.hasFlag).to.not.be.equals(false);
+    });
+
+    it("Should cast 0 as false", () => {
+        const result = plainToClass(Dto, { hasFlag: 0 });
+        expect(result.hasFlag).to.be.equals(false);
+    });
+
+    it("Should cast 1 as true", () => {
+        const result = plainToClass(Dto, { hasFlag: 1 });
+        expect(result.hasFlag).to.be.equals(true);
+    });
+
+    it("Should cast '0' as false", () => {
+        const result = plainToClass(Dto, { hasFlag: "0" });
+        expect(result.hasFlag).to.be.equals(false);
+    });
+
+    it("Should not cast '1' as true", () => {
+        const result = plainToClass(Dto, { hasFlag: "1" });
+        expect(result.hasFlag).to.be.equals(true);
+    });
+
+    it("Should remain real bool", () => {
+        const result = plainToClass(Dto, { hasFlag: true });
+        expect(result.hasFlag).to.be.equals(true);
+    });
+
+});