template

Jul 17, 2023

Imagine this scenario. In Typescript, we have an array that contains some strings and some undefined values.

const rawFruitData: (string | undefined)[] = ["banana", undefined, "apple"]

Now I want to pull out just the fruit strings and remove the undefined from the raw data like so:

const fruits: string[] = rawFruitData.filter((fruit) => fruit)

However, when I try to do this in Typescript I get this error:

Type '(string | undefined)[]' is not assignable to type 'string[]'.
    Type 'string | undefined' is not assignable to type 'string'.
    	Type 'undefined' is not assignable to type 'string'.(2322)

I ran into this problem recently and I used a Type Predicate to help narrow the type.

We can define the Type Predicate like so:

function isFruit(fruit: string | undefined): fruit is string {
	return fruit !== undefined
}

fruit is string is the type predicate, and when isFruit is called on a variable, that variable type will be narrowed to string. isFruit is a user defined type guard. I can use the isFruit method in my filter and get any type errors:

const fruits: string[] = rawFruitData.filter(isFruit);
console.log(fruits) // => ["banana", "apple"]