feat(dataMember): Handle 'strictNullChecks: true' with 'T | null' by adding an additional fieldType prop

[serjKim]

When strictNullCheks: true is used with union type e.g. string | null, typescript emits the 'design:type' metadata as Object rather than string. Example:

class A {
    @dataMember()
    public foo: string | null;
}

const a = deserialize({ foo: 'asd' }, A); // Causes a warning: Deserialization: Primitive type 'asd'' passed to the object for the property 'foo'. It is impossible to determine whether object type is 'any' or 'Object' or 'interface'. Try to avoid 'any' types

Indeed,

...
let A = /** @class */ (() => {
    class A {
    }
    __decorate([
        santee_dcts_1.dataMember(),
        __metadata("design:type", Object) // <-- Object! But must be String
    ], A.prototype, "foo", void 0);
    return A;
})();
const a = santee_dcts_1.deserialize({ foo: 'asd' }, A);

Another one example:

const dec = (target: any, key: string): undefined => undefined;

class A {
    @dec
    public property: string | null;
}

const obj = Reflect.getMetadata('design:type', A.prototype, 'property');

console.log(obj); // Object

There was some discussion microsoft/TypeScript#18509 (comment)
It appears to be the behavior is by design.

I suppose to add an additional fieldType prop to dataMember as a workaround to be able to pass a ctor explicitly:

class A {
    @dataMember({ fieldType: String })
    public foo: string | null;
}

const a = deserialize({ foo: 'asd' }, A);

[serjKim]

@santee @AlexeyVykhrystyuk-GB FYI

[serjKim]

Playground with 4.0.2 version:
https://www.typescriptlang.org/play?strictPropertyInitialization=false#code/MYewdgzgLgBAJgU2DAvDAFFAhgJwOYJQBcMWYAngDQwDWC5J0OAlmHgJQkCuYiAZqwRxUAPhg9+guAG4AULOAAbLBAgwAgjADesmHpgABRMF36ADlwBGi5sjM4QZhDigMYTVnhgAfGGC6KinIAvkA