How can I create an object based on an interface file definition in TypeScript?

If you are creating the "modal" variable elsewhere, and want to tell TypeScript it will all be done, you would use:

declare const modal: IModal;

If you want to create a variable that will actually be an instance of IModal in TypeScript you will need to define it fully.

const modal: IModal = {
    content: '',
    form: '',
    href: '',
    $form: null,
    $message: null,
    $modal: null,
    $submits: null
};

Or lie, with a type assertion, but you'll lost type safety as you will now get undefined in unexpected places, and possibly runtime errors, when accessing modal.content and so on (properties that the contract says will be there).

const modal = {} as IModal;

Example Class

class Modal implements IModal {
    content: string;
    form: string;
    href: string;
    $form: JQuery;
    $message: JQuery;
    $modal: JQuery;
    $submits: JQuery;
}

const modal = new Modal();

You may think "hey that's really a duplication of the interface" - and you are correct. If the Modal class is the only implementation of the IModal interface you may want to delete the interface altogether and use...

const modal: Modal = new Modal();

Rather than

const modal: IModal = new Modal();

If you are using React, the parser will choke on the traditional cast syntax so an alternative was introduced for use in .tsx files

let a = {} as MyInterface;

https://www.typescriptlang.org/docs/handbook/jsx.html

If you want an empty object of an interface, you can do just:

var modal = <IModal>{};

The advantage of using interfaces in lieu of classes for structuring data is that if you don't have any methods on the class, it will show in compiled JS as an empty method. Example:

class TestClass {
    a: number;
    b: string;
    c: boolean;
}

compiles into

var TestClass = (function () {
    function TestClass() {
    }
    return TestClass;
})();

which carries no value. Interfaces, on the other hand, don't show up in JS at all while still providing the benefits of data structuring and type checking.