Typescript doesn't infer type from array.filter() function

[sleek echo]

const startedTests = tests.filter(test => test.thread_id !== null);

tests if of type

{
  thread_id: string | null
}[]

But after the filter() operation

for (
  const thread of startedTests.reduce((accumulator, currentValue) => {
    accumulator.push(currentValue.thread_id); // Argument of type 'string | null' is not assignable to parameter of type 'string'. Type 'null' is not assignable to type 'string'.
    return accumulator;
  }, (<string[]> []))
) {
     // code   
}

Why is typescript considering thread_id as possibly null if tests with a null thread_id value are filtered out?

[zenith idol]

You need to annotate it as a predicate:

const startedTests = tests.filter((test): test is string => test.thread_id !== null)

[sleek echo]

test is an object, wouldn't i need to apply that to thread_id?

[zenith idol]

Oh you are right.

You can do test is { thread_id: string }.

[sleek echo]

it doesn't work

this works

accumulator.push(currentValue.thread_id as string);

[zenith idol]

It does work.

[mild spade]

declare const tests: {
    thread_id: string | null
}[]

const startedTests = tests.filter((test): test is { thread_id: string } => test.thread_id !== null)
//    ^? - const startedTests: {
//        thread_id: string;
//    }[]

[sleek echo]

is the solution i found good as well?

[zenith idol]

No, you generally do not want type assertions, they are unsafe.

[sleek echo]

ok

so from my understanding the predicate will change the variable's type to the test type if compatible, right?

[zenith idol]

Yeah, see https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates for more details.

[sleek echo]

oh alright

it works

thank you