aseemk · November 13, 2016 15:32 · Nov 13, 2016 · Nov 13, 2016 · Nov 13, 2016
diff --git a/typescript-static-and-runtime-typing.ts b/typescript-static-and-runtime-typing.ts
@@ -29,8 +29,7 @@ function parse(val: StaticValueType | null, valType?: StringValueType): StringVa
     }
 }
 
-// These should not compile (but undefined and null do
-// when `--strictNullChecks` isn't enabled):
+// These should not compile (but undefined and null do when `--strictNullChecks` isn't enabled):
 parse();
 parse(undefined);
 parse(null);

diff --git a/typescript-static-and-runtime-typing.ts b/typescript-static-and-runtime-typing.ts
@@ -0,0 +1,65 @@
+type StaticValueType = string | number | boolean;
+type StringValueType = 'string' | 'number' | 'boolean';
+
+function parse(val: string): 'string';
+function parse(val: number): 'number';
+function parse(val: boolean): 'boolean';
+function parse<T extends StringValueType>(val: null, valType: T): T;
+function parse(val: StaticValueType | null, valType?: StringValueType): StringValueType {
+    if (val == null) {
+        if (valType == null) {
+            throw new Error();
+        }
+        return valType;
+    }
+
+    const t = typeof val;
+
+    switch (t) {
+        case 'boolean':
+        case 'number':
+        case 'string':
+            // TODO: Ideally, this cast wouldn't be necessary!
+            // It ties this code to the StringValueType type union.
+            // It'd be nice to statically verify we've exhaustively checked.
+            return t as StringValueType;
+
+        default:
+            throw new Error('Should never happen!');
+    }
+}
+
+// These should not compile (but undefined and null do
+// when `--strictNullChecks` isn't enabled):
+parse();
+parse(undefined);
+parse(null);
+parse({});
+parse([]);
+parse(parse);
+
+// These should compile, and the type should be correctly inferred!
+let strType = parse('');
+let numType = parse(0);
+let boolType = parse(false);
+let verifyStrType: 'string' = strType;
+let verifyNumType: 'number' = numType;
+let verifyBoolType: 'boolean' = boolType;
+
+// These should not:
+strType = parse(0);
+numType = parse(false);
+boolType = parse('');
+
+// These should compile, and the type should be correctly inferred too!
+strType = parse(null, 'string');
+numType = parse(null, 'number');
+boolType = parse(null, 'boolean');
+verifyStrType = strType;
+verifyNumType = numType;
+verifyBoolType = boolType;
+
+// These should not:
+strType = parse(null, 'boolean');
+numType = parse(null, 'string');
+boolType = parse(null, 'number');