Interface type check with Typescript

Question

How to find out at runtime if a variable of type any implements an interface? 

interface A{
    member:string;
}

var a:any={member:"foobar"};

if(a instanceof A) alert(a.member);

If you enter this code in the typescript playground, the last line will be marked as an error, "The name A does not exist in the current scope". I even tried changing the variable declaration but even that did not work out well. I could not find anything useful on the web and so I am posting this question.

I understand that javascript as a dynamic language has no concept of interfaces. Is there any way to type check for interfaces?

The typescript playground's autocompletion reveals that typescript even offers a method implements. How can I use it ?

Answer 1

You can achieve what you want without the instanceof keyword as you can write custom type guards now:

interface A{
    member:string;
}

function instanceOfA(object: any): object is A {
    return 'member' in object;
}

var a:any={member:"foobar"};

if (instanceOfA(a)) {
    alert(a.member);
}