abi
abi•2y 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.
marvinh.•2y 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
abiOP•2y 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
𝕒𝕣𝕤𝕙•2y ago

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