Make type validator give back right type

Issue

This is my code:

function isArray(value) {
  return Array.isArray(value)
}

function someFunction(value: Array<any> | boolean) {
  if (isArray(value)) {
    console.log(value.length)
  } else {
    console.log(value)
  }
}

And I got this error:

In my opinion, I think I've checkd type of value and make sure it is a array. But compiler of typescript doesn't recognize. How could I reslove this?

Solution

function isArray(value: any): value is any[] {
  return Array.isArray(value)
}

You need to add a return type to isArray, making it a type predicate.

More info here: https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates

Answered By - Anastasia