js-main.js

// @ts-check
import {Person} from './person.js';

const clippy = new Person('Clippy',
    new Date(1996, 11 - 1, 19));
const rocky = new Person('Rocky',
    9879879879879877);

person.d.ts

export interface PersonStruct {
    name: string;
    birthday: Date;
}
export class Person {
    name: string;
    birthday: Date;
    constructor(name: string, birthday: Date);
    getAge(from: Date): number;
}
export const example: Person;

person.js

// @ts-check
export class Person {
    constructor(name, birthday) {
        this.name = name;
        this.birthday = birthday;
    }
    /** Naive implementation of years */
    getAge(at=new Date()) {
        return at.getFullYear() - this.birthday.getFullYear();
    }
}

// I would think that the following should report an error
export const example = new Person('Unix Epoch Man', 0);

// I would expect this type would be found in corresponding
// declaration file
/** @type {PersonStruct} */
var p = {name: '', birthday: new Date(0)};

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