Jest typescript check for type

Question

lets say i have the following interface:

export interface CMSData {
    id: number;
    url: string;
    htmlTag: string;
    importJSComponent: string;
    componentData: ComponentAttribute[];
}

Then i have a method that returns an array of this object type:

public async GetContent(url: string): Promise<CMSData[]>{
    const response = await super.get<ICMSContentData[]>(url, {});
    try {
        if (response?.parsedBody) {
            return this.ProcessResponse(response.parsedBody);
        } else {
            this.handleHTTPError(new Error("Error"));
            return [];
        }

    } catch (e) {
        this.handleHTTPError(e);
        return [];
    }

}

Then i want to test that this is the case so i write the following test:

import {ContentIOService} from "..";
import {CMSData} from "../IOServices/ContentIOService";

require('es6-promise').polyfill();
require('isomorphic-fetch');

test('Get Content', async () => {
    const service = ContentIOService.getInstance();
    const data = await service.GetContent("https://1c7207fb14fd3b428c70cc406f0c27d9.m.pipedream.net");
    console.log(data)
    expect(data).toBeInstanceOf(CMSData[]);
});

However here i get the following error:

'CMSData' only refers to a type, but is being used as a value here.

So how can i test that the data i get back is valid and of the right type?

Answer 1

If the type you're looking for is a call, the .toBeInstanceOf(Class) method accepts a parameter which MUST BE a JavaScript class constructor instead of TS type.

You should let TSC check whether you receive the correct type of data at compile time. Code written inside test suites and test cases is executed at runtime, .toBeInstanceOf(Class) is a runtime check, NOT compiler time.

At runtime, you may want to use expect.objectContaining(object) matches any received object that recursively matches the expected properties.