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I would like a test to expect a Thrown Error in case a class property this.url is not defined.

What am I doing wrong here ?

 it('should throw an error if url is not provided by default', async () => { // Given const Service = require('./services/websocket') // When const Websocket = new Service() // Then expect(Websocket.url).toBeUndefined() expect(Websocket).toThrowError('Websocket URL is not provided') }) 
// services/websocket.js class Websocket { constructor () { this.url = undefined if (!this.url) { throw new TypeError('Websocket URL is not provided') } } } module.exports = Websocket

Jest error message:

 FAIL terminal.test.js Websocket service provider ✕ should throw an error if url is not provided by default (2 ms) ● Websocket service provider › should throw an error if url is not provided by default TypeError: Websocket URL is not provided 4 | 5 | if (!this.url) { > 6 | throw new TypeError('Websocket URL is not provided') | ^ 7 | } 8 | } 9 | } at new Websocket (services/websocket.js:6:13) at Object.<anonymous> (terminal.test.js:7:23)
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  • Does this answer your question? How to test the type of a thrown exception in Jest Commented Nov 15, 2022 at 9:00
  • Well, the websocket object itself is not throwing an error, but the call to new Service() is. This error itself is not handled by your test. Nevertheless it's a little bit strange that your class is named Service and not Websocket as seen in the class definition. Commented Nov 15, 2022 at 9:00
  • Oh, nevermind. I just saw your export. This answers my confusion. Commented Nov 15, 2022 at 9:02
  • @aspirinemaga you tried try-catch block to catch error and then did the assertion? Commented Nov 15, 2022 at 9:07
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    expect(Websocket).toThrowError('Websocket URL is not provided') - you expect accessing the variable to throw an error? Even if you did, that access would be done before expect got called, in an attempt to resolve the value to pass to it, so couldn't be caught. Look again at the examples in the docs, and think about how JS control flow works: jestjs.io/docs/expect#tothrowerror. Commented Nov 15, 2022 at 10:29

4 Answers 4

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Another way to assert the error in a class which I find it a more proper way:

it('should throw an error if url is not provided by default', () => { // Given const Service = require('./websocket') // When const Module = () => new Service() // When expect(Module).toThrowError('Websocket URL is not defined') }) 
class Websocket { constructor () { this.url = process.env.WS_URL if (!this.url) { throw new Error('Websocket URL is not defined') } } } module.exports = Websocket
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 it('should throw an error if url is not provided by default', (done) => { // Given const Service = require('./services/websocket') // When try { const Websocket = new Service() } catch(err){ expect(Websocket.url).toBeUndefined() expect(err.message).toBe("Websocket URL is not provided") done() } throw new Error('it did not catch url is not provided error') })
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-1

I've managed to make it work.

 it('should throw an error if url is not provided by default', () => { // Given const Service = require('./services/websocket') // When let Module try { Module = new Service() } catch (error) { Module = error } expect(Module).toBeInstanceOf(Error) expect(Module.message).toBe('Websocket URL is not defined') }) it('should pick up a WS url from env file automatically', () => { process.env.WS_URL = 'ws://dummyurl' const Service = require('./services/websocket') const Module = new Service() expect(Module.url).toEqual(process.env.WS_URL) }) 
// services/websocket.js class Websocket { constructor () { this.url = process.env.WS_URL if (!this.url) { throw new Error('Websocket URL is not defined') } } } module.exports = Websocket
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-1

I think you're probably looking for something more like

it('should throw an error if url is not provided by default', () => { const Service = require('./services/websocket') expect(new Service()).toThrowError() })

The want to check for the URL to be undefined is understandable, but I don't believe anything would execute after the error unless you catch and discard it, so the lack of it being set shouldn't be a show stopper.

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Could you recommend some different approach of writing those checks in class constructor instead ? I do understand now that an error is occured before the expect class, without trycatch I wouldn't be able to assert it.
I think your original class implementation is good. Like you probably don't want to have execution continue if you don't have everything you need to be able to build the connection. An alternative might be to delay the check. For instance, you could delay the check until right before you make the connection. So you would add a connect method to your Websocket class and call that in the caller's scope. Your constructor then only assigns the information it's given if it's given it. Then in the connect method, right before you make the ws conn, verify and throw there.
``` class Websocket { constructor () { this.url = process.env.WS_URL } connect() { if (!this.url) { throw new Error('Websocket URL is not defined') } } } module.exports = Websocket``` ``` //example const service = new Service(); service.connect() // throws since we didn't provide a URL ```

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