Function hasValueAtKey

hasValueAtKey<const K extends string | number | symbol, const V>(
    k: K,
    v: V,
): <T>(
    a: T & { [k in string | number | symbol]: unknown },
) => a is T & { [k in string | number | symbol]: V }

Returns a function that can be used to filter down objects to the ones that have a specific value V under a key k.

Type Parameters

const K extends string | number | symbol
const V

Parameters

k: K
v: V

Returns <T>(
a: T & { [k in string | number | symbol]: unknown },
) => a is T & { [k in string | number | symbol]: V }

Example

type File = { type: "image", imageUrl: string } | { type: "pdf", pdfUrl: string };
const files: File[] = [];

const imageFiles = files.filter(file => file.type === "image");
files[0].type // In this case, TS will still treat it  as `"image" | "pdf"`

const filesWithUrl = files.filter(hasValueKey("type", "image" as const));
files[0].type // TS will now know that this is "image"
files[0].imageUrl // TS will know this is present, because already it excluded the other union members.