Function hasValueAtKey

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

    Returns <T>(a: T & { [k in K]: unknown }) => a is T & { [k in K]: 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.

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