How to use Jest to test file download?

Question

I have some code as below:

/* global document */ /* global window */ /* global Blob */ import FileSaver from 'file-saver'; export const createDownloadFromBlob = (blob, filename, extension) => { FileSaver.saveAs(blob, `${filename}.${extension}`); }; export const createDownload = (content, filename, extension) => { createDownloadFromBlob(new Blob([content], { type: 'application/octet-stream' }), filename, extension); };

I want to use Jest to unit-test these two methods, but I don't know where to start. Any help would be appreciated.

Andreas Köberle · Accepted Answer · 2017-06-22 06:59:41Z

I would mock out FileSaver with a spy:

import FileSaver from 'file-saver'; jest.mock('file-saver', ()=>({saveAs: jest.fn()}))

As you cant compare Blobs I would mock this as well:

global.Blob = function (content, options){return ({content, options})}

now you can run your test and use expect like this

createDownload('content', 'filename', 'extension') expect(FileSaver.saveAs).toHaveBeenCalledWith( {content:'content', options: { type: 'application/octet-stream' }}, 'filename.extension' )

David Dehghan · Accepted Answer · 2019-01-31 20:31:26Z

In Typescript: If you create a Blob with a ArrayBuffer or binary data then you need handle that case separately than strings.

import * as CRC32 from 'crc-32'; (window as any).global.Blob = function(content, options) { // for xlxs blob testing just return the CRC of the ArrayBuffer // and not the actual content of it. if (typeof content[0] !== 'string') { content = CRC32.buf(content); } return {content: JSON.stringify(content), options}; };

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