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Just look at this typescript code:

lib.ts

interface Human 
 name: string;
 age: number;


export default class HumanFactory 
 getHuman(): Human 
 return 
 name: "John",
 age: 22,
 
 

index.ts

import HumanFactory from "./lib";

export class Foo 
 human: any;

 constructor() 
 const factory = new HumanFactory();
 this.human = factory.getHuman();
 

 diffWithError(age: number): number 
 return age - this.human.name;
 

 diffWithTypingAndAutocoplete(age: number): number 
 const factory = new HumanFactory();
 return age - factory.getHuman().name;
 

The problem in "human" property of "Foo" class. I can't define type of this variable as "Human" interface from lib.ts.

In method "diffWithError" I make an error - use number "age" and string "name" in arithmetic operation, but neither IDE nor ts compiler know about this, because in this context, type of "this.human.name" is "any"

In method "diffWithTypingAndAutocoplete" I just use method "getHuman". IDE and compiler know about type of method result. This is "Human" interface and field "name" are "string". This method trigger an error when compiling sources.

I found this problem when I tried import .d.ts file of JS lib and I don't have ability to export needed interface. Can I somehow define valid type of "human" property without copy and paste code of "Human" interface whenever i want to define type (and without inline type definitions, like name: string, age: number ).

I don not want to create instances of not exported classes, I just want type checking and autocomplete.

P.S. I try write this:

human: Human

compiler trigger an error: "error TS2304: Cannot find name 'Human'" (expected behavior)

P.S.S I try to do this with triple slash directive:

///<reference path="./lib.ts" />

but this not working too.

Sorry for my poor english and thanks for answers

If you can't change lib.ts you can "query" return type of getHuman function. It is a bit tricky because typescript currently doesn't provide any straight forward method for this:

import HumanFactory from "./lib";

const dummyHuman = !true && new HumanFactory().getHuman();
type Human = typeof dummyHuman;

export class Foo 
 human: Human;

 // ...

!true && is used to prevent new HumanFactory().getHuman() execution.

I found a solution!

I make file human-interface.ts with this content:

import HumanFactory from './lib';

const humanObject = new HumanFactory().getHuman();
type HumanType = typeof humanObject;

export default interface Human extends HumanType 

Import of this interface in main file not execute creation of "HumanFactory" and type checking work properly.

Thanks for idea with typeof

You need to export Human so that it is visible - and usable - from index.ts as well (as HumanFactory). Do not use default exports but "named exports" i.e. try this

export interface Human 
 name: string;
 age: number;


export class HumanFactory 
 getHuman(): Human 
 return 
 name: "John",
 age: 22,
 
 

In index.ts

import Human, HumanFactory from "./lib";

** EDIT **

If you cannot change lib.d.ts then redefine Human and use double-casting i.e.

import HumanFactory from "./lib";

interface Human 
 name: string;
 age: number;


export class Foo 
 human: Human; // <= change here

 constructor() 
 const factory = new HumanFactory();
 this.human = factory.getHuman() as any as Human; // <= double casting 
 

 diffWithError(age: number): number 
 return age - this.human.name;
 

 diffWithTypingAndAutocoplete(age: number): number 
 const factory = new HumanFactory();
 return age - factory.getHuman().name;
 

This was made easier with the introduction of the ReturnType<> static type in TypeScript 2.8.

import HumanFactory from "./lib";
type Human = ReturnType<typeof HumanFactory.prototype.getHuman>

See also https://stackoverflow.com/a/43053162