JavaScript

The class feature of modern JavaScript offers a good way to group a fact constructor with a set of specification functions. The type is added as a prototype field.

class Tag {
    constructor(
        name
    ) {
        this.type = Tag.Type;
        this.name = name;
    }
}
Tag.Type = "Blog.Tag";

A fact can then be created with the new operator.

const tagReact = await j.fact(new Tag("React"));