TypeScript initiate an empty interface object

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' });