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1. let, const and ... var

Islam Sayed on February 11, 2019

ES6 has introduced some new syntax features. One of them was key words const, and let to declare variables. Let's talk about why they are preferred...

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Adrian • Feb 11 '19

var is useful with Jest.

try {
  await foo()
} catch (e) {
  var error = e
}
expect(error.message).toMatch(/foo/)

Christian C. Salvadó • Feb 12 '19

Honestly, I would simply:

await expect(foo()).rejects.toThrow(/foo/)

Since foo is an async function, it implicitly returns a promise so you can use the .resolves / .rejects matchers and then use the toThrow method, and this accepts a regex or string to match the error message property. IMHO it reads more semantically correct.

Cheers,

Mihail Malo • Feb 13 '19

The toThrow API is quite weird.
If you pass a string, it matches it anywhere, so 'foo' and /foo/ is the same.
And if you want to strictly match the whole message, you need to do

.toThrow(/^literal message$/) // RegEx with ends
.toThrow(new Error('literal message')) // The Error class does NOT matter in this case.

If you actually care about the constructor, you have to pass just it.

.toThrow(SpecialError)

I use

expect(badFunc).toThrowErrorMatchingInlineSnapshot()

a lot nowadays.

A. • Feb 12 '19

Any reason to not put the expect inside the catch bloc ?

Mihail Malo • Feb 12 '19 • Edited

Yes. If it doesn't throw assertion wouldn't run, and the test would pass.
You'd have to do

expect.assertions(1)
try {
  await foo()
} catch (e) {
  var error = e
  expect(error.message).toMatch(/foo/)
}

Personally, I'd just go with

await expect(foo()).rejects.toThrow('foo')
// Or, if final in the test:
return expect(foo()).rejects.toThrow('foo') // doesn't require `async` `it`

(No need for expect.assertions(1) since the expect runs synchronously inline)

Islam Sayed • Feb 11 '19

Could you please explain why you use only var with jest?

Adrian • Feb 11 '19

Oh, no. I only ever use var in this scenario. Lets me skip out of the block scope without a separate variable declaration. And if no error happens, the variable is just undefined.

Islam Sayed • Feb 11 '19

Aha :)
So here is a place where var is still useful 😉

Mihail Malo • Feb 12 '19 • Edited

@adrianhelvik
Did you mean:

let error
try {
  await foo()
} catch (e) {
  error = e
}
expect(error.message).toMatch('foo') // you can use string literals if you don't have a complex RegEx