Why doesn't TypeScript narrow down any type in this type?

Question

The following example confuses me:

class MyClass
{
    public MyProp = "";
}

var object: any = null;

if (object instanceof MyClass)
    console.log(object.MyProp, object.NonExistant); // <-- No error, no intellisense

Why doesn't the type guardian give me the correct type in the checked context?

Answer 1

Custom types of guards and instanceofType of protection does not narrow the types that are any(including union and intersection c any).

The only thing that will narrow down the anytype defender is to use validation typeofand validation for the primitive string, booland number:

var something: any;
if (typeof something === "string")
{
    something.NonExistant(); // <- Error, does not exist on `string`
    something.substr(0, 10); // <- Ok
}

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