How to extend JQuery functions in TypeScript

Question

I'm rewriting some JS code on TypeScript and encounter with problems with module import. For example, I want to write my toggleVisiblity function. Here is code:

/// <reference path="../../typings/jquery/jquery.d.ts" />

import * as $ from "jquery";

interface JQuery {
    toggleVisibility(): JQuery;
}

$.fn.extend({
    toggleVisibility: function () {
        return this.each(function () {
            const $this = $(this);
            const visibility = $this.css('visibility') === 'hidden' ? 'visible' : 'hidden';
            $this.css('visibility', visibility);
        });
    }
});

const jQuery = $('foo');
const value = jQuery.val();
jQuery.toggleVisibility();

But the problem is that for unknown reason toggleVisibility is not added to JQuery interface thus I get an error Property 'toggleVisibility' does not exist on type 'JQuery'., although it sees other methods (val, each and so on).

Why is it not working?

Answer 1

Try putting the

interface JQuery {
    toggleVisibility(): JQuery;
}

Inside a seperate file without import/export statements. This works for me. Though it wold be interesting to know why.

EDIT: There is an excellent explanation for this behaviour in this answer to post: How to extend the 'Window' typescript interface