Function hasValueAtKey

  • hasValueAtKey<const K, const V>(k, 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
    })

      • <T>(a): a is T & {
            [k in K]: V
        }

      • Type Parameters

        • T

        Parameters

        • a: T & {
              [k in K]: unknown
          }

        Returns 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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