// A *self-contained* demonstration of the problem follows... // Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc. interface TypeA { test: "test1" | "test2" | "test3" } function fn(param: TypeA) { } // Check pass fn({ test: "test1" }); // The same object, but passed by object reference let obj = { test: "test1" }; /**  * Error:(26, 4) TS2345: Argument of type '{ test: string; }' is not assignable to parameter of type 'TypeA'.  * Types of property 'test' are incompatible.  * Type 'string' is not assignable to type '"test1" | "test2" | "test3"'.  */ fn(obj);