abi•15mo ago

Simple type narrowing/type guard/assertion (?)

Is there really no simpler way of doing this without any and without external deps?

export function isFooBarBaz(
  value: unknown,
): value is { Foo: { Bar: { Baz: string } } } {
  return typeof value === "object" &&
    value !== null &&
    "Foo" in value &&
    typeof value.Foo === "object" &&
    value.Foo !== null &&
    "Bar" in value.Foo &&
    typeof value.Foo.Bar === "object" &&
    value.Foo.Bar !== null &&
    "Baz" in value.Foo.Bar &&
    typeof value.Foo.Bar.Baz === "string";
}

export function isFooBarBaz(
  value: unknown,
): value is { Foo: { Bar: { Baz: string } } } {
  return typeof value === "object" &&
    value !== null &&
    "Foo" in value &&
    typeof value.Foo === "object" &&
    value.Foo !== null &&
    "Bar" in value.Foo &&
    typeof value.Foo.Bar === "object" &&
    value.Foo.Bar !== null &&
    "Baz" in value.Foo.Bar &&
    typeof value.Foo.Bar.Baz === "string";
}

(What's the exact name for what I'm doing here btw? I confuse the terms.)

3 Replies

marvinh.•15mo ago

If you want to simplify that a little bit you can drop the "KEY" in my.obj.prop checks there since the typeof expression after that takes care of that. I've seen folks using libraries like zod or @badrap/valita for building up schemas which can be used to do validate an unknown value if it matches a type.

abi•15mo ago

yeah, i'm actually resorting to zod now, even though i feel like i shouldn't need to :/ but i do have pretty deep and complex objects

𝕒𝕣𝕤𝕙•15mo ago

Beast's snippet https://discord.com/channels/684898665143206084/684898665151594506/1108766637164859482 my snippet https://discord.com/channels/684898665143206084/684898665151594506/1108779575351201882