TypeScript - multiple assertions

Question

I am trying to assert that multiple values are not undefined using TypeScript's asserts feature. While I am able to assert a single value, I am not able to assert multiple values by a single statement. See below.

function assert1(a1: any): asserts a1 {
  if (a1 === undefined) throw new Error()
}
function assert2(a1: unknown, a2: unknown) {
  assert1(a1)
  assert1(a2)
}

const foo = () => Math.random() < 0.5 ? 4999 : undefined

const a = foo()
const b = foo()

assert1(a)
console.log(a + 10) // works as expected, no errors


assert2(a, b)
console.log(a + b) // const b: 4999 | undefined, Object is possibly 'undefined'.(2532)

I spent quite a long time with this but to no avail. Is it possible to make this work? Or do I have to stick with the traditional:

if (!a || !b || !c || !d ...) {
  throw ...
}

Thanks for your help and insights.

To assert a value a function must have an asserts signature. Your assert2 has no, so that function call asserts nothing. — zerkms
– zerkms, Commented Jan 17, 2020 at 10:31
..also you should check the type using typeof a1 === 'undefined'; unfortunately in JS undefined can be redefined... to assign to undefined you can use a = void 0 — Bruno Grieder
– Bruno Grieder, Commented Jan 17, 2020 at 10:35
Unfortunately I am not aware of any solution to aggregate your assertions. Consider writing one assertNotUndefined and use it for each variable in your case a and b. — r3dst0rm
– r3dst0rm, Commented Jan 17, 2020 at 10:38

ford04 · Accepted Answer · 2020-01-17 11:00:52Z

5

Instead of asserting one single value, you also can assert conditions, which are truthy for the rest of the scope (more infos here):

function assert(condition: any, msg?: string): asserts condition {
    if (!condition) throw new Error(msg)
}

const foo = () => Math.random() < 0.5 ? 4999 : undefined

const a = foo()
const b = foo()

assert(a !== undefined && b !== undefined) // assert a and b are defined

console.log(a + b) // works; a: 4999, b: 4999

Playground sample

answered Jan 17, 2020 at 11:00

ford04

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1 Comment

Jan Legner Over a year ago

Thanks, have not thought of that.

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