Can a self-referential type truly exist?

There is a function that takes in a configuration object containing color definitions.

For example:

useColors({
  colors: {
    RED: { hex: 0xff0000 },
    GREEN: { hex: 0x00ff00 },
    BLUE: { hex: 0x0000ff }
  },
  doSomethingWithColor(getColor) {
    getColor('RED') // should accept "RED" | "GREEN" | "BLUE"
  }
})

The function also accepts another function doSomethingWithColor, and the callback parameter type of this function should be a combination of the colors defined in the configuration object itself.

Can TypeScript achieve this?

typescript

Answer №1

To implement this feature, you can leverage the power of generics:

function useColors<Palette>(config: {
  colors: Palette,
  performActionWithColor: (getColor: <Key extends keyof Palette>(color: Key) => Palette[Key]) => void;
}) {
  const getColor = <Key extends keyof Palette>(c: Key) => config.colors[c];
  config.performActionWithColor(getColor);
}

By utilizing the code snippet above, the objects within your Palette can have distinct structures:

useColors({
  colors: {
    RED: { hexR: 0xff0000 },
    GREEN: { hexG: 0x00ff00 },
    BLUE: { hexB: 0x0000ff }
  },
  performActionWithColor(getColor) {
    const value = getColor("GREEN") // type of 'value' is { hexG: number }
  }
})

Furthermore, you have the option to confine the values in the Palette to a specific format using a constraint like so:

function useColors<Palette extends Record<string, { hex: number }>>(config: {
  colors: Palette,
  performActionWithColor: (getColor: <Key extends keyof Palette>(color: Key) => Palette[Key]) => void;
}) {
  const getColor = <Key extends keyof Palette>(c: Key) => config.colors[c];
  config.performActionWithColor(getColor);
}

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