Declare Typescript Interface for non Typescript class

Question

Is it possible to declare a TypeScript interface for a plain JavaScript class?

e.g.

function Foo(bar)
{
  this.bar=bar;
}

var x=new Foo("test"); // x is shown as any

I'd like to declare an interface for Foo:

interface IFoo
{
  bar: string;
}

But I can't figure out how to declare it.

function Foo(bar: string) : IFoo
{
  this.bar=bar;
}

Gives me "'Foo' declared a non-void return type, but has no return expression."

(I don't want to rewrite Foo as a TypeScript class.)

Maybe similar question: stackoverflow.com/questions/3710275/… — jsmorph
– jsmorph, Commented Sep 14, 2013 at 21:30
As @basarat says, separate your JavaScript code from TypeScript code, or convert your JavaScript code into TypeScript (which will compile into a version similar to your original JavaScript code). — Stephen Chung
– Stephen Chung, Commented Sep 18, 2013 at 3:09

basarat · Accepted Answer · 2013-09-14 22:59:56Z

4

You can simply declare it to be a class :

declare class Foo{
    bar:string;
    constructor(bar:string);
}

var x=new Foo("test"); // x of type foo
x.bar="lala";

answered Sep 14, 2013 at 22:59

basarat

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3 Comments

This will not compile - it gives me a Duplicate identifier 'Foo'. for function Foo(bar) {...}!

You need to put the JavaScript code in JavaScript. You cannot type in typescript and tell the compiler that this is a class not a function

Thanks, I was trying to integrate a pure Javascript library with typescript and this did the trick.