Let's look at an illustration:
type Colors = {
light: 'EC3333' | 'E91515'
dark: '#100F0F' | '140F0F'
}
interface Palette {
colorType: keyof Colors
color: Colors[keyof Colors]
}
Is it possible for the type of the color property to be dependent on the colorType property?
This means that when selecting colorType light, the type of color should be 'EC3333' | 'E91515'
Representing Palette as a single non-generic interface is not possible, but you can create a union of each desired combination of colorType and color:
type Palette = {
colorType: "light";
color: "EC3333" | "E91515";
} | {
colorType: "dark";
color: "#100F0F" | "140F0F";
}
To automatically generate this union type from Colors, you can map over the keys of Colors and then immediately index into the resulting mapped object type:
type Palette = { [K in keyof Colors]: {
colorType: K
color: Colors[K]
} }[keyof Colors];
By using IntelliSense, you can verify that this results in the same type as before, with the added benefit of automatic updates if Colors change.
With this union type, you can now use Palette and ensure that the constraints are enforced and understood:
const badPalette: Palette = {
colorType: "dark",
color: "E91515"
} // error!
const goodPalette: Palette = {
colorType: "dark",
color: "140F0F"
} // okay
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