Typescript: Testing file uploads with Supertest

[Tessmore]

If you have an API with an end-point that accepts files, and you want to test it then multiple approaches work:

• Create and commit files to repository. This is probably the easiest way to test formats such as Excel/Powerpoint. But you might not want to commit the files...
• Transform data to a Buffer and use that instead. For regular objects this is quite simple. For Excel (or other formats) you probably want to use a library to generate the Buffer.

/**
 * Helper to "transform" regular data to an API end-point 
 * that accepts file uploads
 *
 * @param agent: Supertest agent that is logged in
 * @param data: Regular data object
 * @param someValue: Example of other static value(s) you want to pass along with the "file"
 */
export async function transformData(
    agent: SuperTest<Test>,
    data: SomeInterface,
    someValue: string
): Promise<any> {
    // Convert data to a Buffer
    const dataBuffer = Buffer.from(JSON.stringify(data), 'utf-8');

    return (
        await agent
            .post(`/transform`)
            .set("Content-Type", "multipart/form-data")
            .field("someValue", someValue)
            // Set filename in case back-end does validation (e.g. only accepts files with JSON extension)
            .attach("files[]", dataBuffer, { filename: "tmp-file.json" })
            .expect(201)
    ).body;
}