Avoiding type merging in intricate distributed union TypeScript type?

At my TypeScript playground, I've encountered some issues with a large entrypoint. Although a simplified version seems to work, the actual function 'convert' is causing problems. The main issue arises when calling nested functions like 'convertFontWithFontForgeNode' and 'convertImageWithImageMagickNode'. There are type errors being displayed that suggest an incompatibility between ImageMagick and FontForge types.

export async function convert<I extends ConvertInputFormat>(
  source: Convert<I>,
) {
  // code block here
}

The error messages indicate a mismatch in format types between ImageMagick and FontForge. By reducing the number of enum values for the 'format', the issue seems to be resolved. This could be due to overlapping values between FontFormat and ImageMagick types causing conflicts.

The goal is to have different formats supported within the 'convert' function, with each format having its own properties and configurations. However, attempts to handle these variations using 'ConvertOutputFormat[I]['extend']' seem to be failing.

Additional complexity arises from the need to pass types down to nested functions like 'convertFontWithFontForgeNode' using Zod schemas. These schemas directly correspond to TypeScript types but may not be fully compatible with the overarching 'convert' function.

Overall, the objective is to ensure seamless conversion functionality for various input-output pairs while maintaining clear distinctions between formats and associated properties.

typescriptzod

Answer №1

To achieve this, utilize conditional types instead of using union types. Although your playground was too intricate to completely fix, a simplified version can be crafted as follows:

const FONT_FORMAT = ['ttf', 'woff', 'svg'] as const
type FontFormat = (typeof FONT_FORMAT)[number]

const IMAGE_MAGICK_INPUT_FORMAT = ['jpg'] as const
type ImageMagickInputFormat = (typeof IMAGE_MAGICK_INPUT_FORMAT)[number]

const IMAGE_MAGICK_OUTPUT_FORMAT = ['jpg'] as const
type ImageMagickOutputFormat = (typeof IMAGE_MAGICK_OUTPUT_FORMAT)[number]

async function convert<InputFormat extends FontFormat | ImageMagickInputFormat>(source: {
  input: {
    format: InputFormat,
    file: { path: string }
  },
  output: {
    format: InputFormat extends FontFormat ? FontFormat : ImageMagickOutputFormat,
    file: { path: string }
  }
}) {
  // TODO: implement
}

This modification will make the example input operate correctly like in this TS playground simulation.

While functional, it is recommended to separate functionalities into individual functions rather than combining them into one. This approach significantly simplifies the types.

A comprehensive mutually exclusive union of types you desire is not available in Typescript. The proposed addition was never integrated into the language. There have been successful workarounds for shallow types and even a package called "ts-xor" that introduces the type XOR<A, B, C, D, E, F, ...>. However, none seem to support complex nested types at the moment.

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