Disable incorrect tslint warning

TSLint reports that the result of a decoded object field can't be undefined in
this situation. The result can definitely be undefined when the object field is
optional and not present.

Author: Elias Mulhall

src/decoder.ts

@@ -241,6 +241,7 @@ export class Decoder<A> {
           if (decoders.hasOwnProperty(key)) {
             const r = decoders[key].decode(json[key]);
             if (r.ok === true) {
+              // tslint:disable-next-line:strict-type-predicates
               if (r.result !== undefined) {
                 obj[key] = r.result;
               }

test/json-decode.test.ts

@@ -251,6 +251,16 @@ describe('object', () => {
       result: {a: true, b: 'hats'}
     });
   });
+
+  it('ignores optional fields that decode to undefined', () => {
+    const decoder = object({
+      a: number(),
+      b: optional(string())
+    });
+
+    expect(decoder.run({a: 12, b: 'hats'})).toEqual({ok: true, result: {a: 12, b: 'hats'}});
+    expect(decoder.run({a: 12})).toEqual({ok: true, result: {a: 12}});
+  });
 });
 
 describe('array', () => {