How can the generic type of a const variable with inferred type be narrowed down in Typescript?

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In Typescript, the generic type of a const variable with inferred type can be narrowed down using the as keyword or the is type guard.

For example, suppose we have a const variable arr with inferred type Array<number | string>:

const arr = [1, "two", 3, "four"] // inferred type: Array<number | string>

We can narrow down the generic type to Array<string> using the as keyword:

const stringArr = arr.filter((val) => typeof val === "string") as Array<string>

Alternatively, we can use the is type guard to narrow down the generic type:

function isString(val: any): val is string {
  return typeof val === "string"
}

const stringArr = arr.filter(isString) // inferred type: Array<string>