An interface does not define an initial value for actual objects, but defines the requirements an object must meet for it to be an implementation of that interface.

You should create a factory of some kind that creates objects of type Article, this could be as simple as this function:

const emptyArticle = (): Article => ({
    slug: '',
    title: '',
    description: '',
    body: '',
    tagList: [],
});

You could additionally create a function that accepts an object that is a Partial<Article> - as described here. This enables you to pass a subset of initial values to the function. This example uses emptyArticle defined above:

const createArticle = <T extends Partial<Article>>(initialValues: T): Article & T => {
    return Object.assign(emptyArticle(), initialValues);
};

// Initialize a new article with a certain slug:
createArticle({ slug: 'my-awesome-article' });

I have hundreds of empty variable declarations like this, and I can't know at this point in the code the "type" of variable being stored later,

You don't know what sort of data you're planning on putting in the variable?

This behavior is intentional.

The excess property check is not performed when the target is an empty object type since it is rarely the intent to only allow empty objects.

Actually, you can assign {a: 1} to B, the other answers here are mostly wrong.

You have stumbled upon another slightly confusing quirk in TypeScript, namely that you can not directly assign an object literal to a type where the object literal contains other properties than the one specified in the type.

However, you can assign any existing instance of an object to a type, as long as it fulfill the type.

For example:

interface Animal {
    LegCount: number;
}

let dog: Animal = { LegCount: 4, Fur: "Brown" }; // Nope

var myCat = { LegCount: 4, Fur: "Black" };
let theCat: Animal = myCat; // OK

This constraint is simply ignored whey you have a type that is empty.

Read more here and here.

A later answer from the Typescript team is available on GitHub.

You can use type assertion for that:

let map: MapObj = {} as MapObj;

Or simply:

let map = {} as MapObj;

(code in playground)

Edit

The type you have for MapObj is a type for a function, so something like this:

let map: MapObj = function (s: string) {
    return {};
};

If you just want an object which maps between strings (key) to TaskDaylist (values), then it should look like this:

interface MapObj {
    [s: string]: TaskDaylist;
}

More about it in Indexable Types.

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();