Let's say I have the following type declarations:
type Foo = 'a' | 'b' | 'c';
type Bar = 'a' | 'b' ;
Can we define Bar as a subset of Foo?
I know it's always possible to define Foo as a superset of Bar, but in my scenario, expressing it the other way around would align better with the context.
If you want to exclude certain values from a predefined conditional type, you can easily achieve this by using the Exclude method:
type Colors = 'red' | 'blue' | 'green';
type PrimaryColors = <b>Exclude</b><Colors, 'green'>;
const PrimaryColors = 'red';
It's worth noting that the following syntax works as well, even though it might seem odd initially:
type PrimaryColors = Exclude<Colors, 'yellow'>
Check out the example on the TypeScript playground for more clarity.
Furthermore, you can combine exclusion with index types to achieve interesting results:
type Colors = 'red' | 'blue' | 'green';
type ColorObject = { green: boolean }
type PrimaryColors = Exclude<Colors, keyof ColorObject>
const primaryColorVar: PrimaryColors = "red";
For example:
let newVariable = Omit<OldVariable, 'x'>
(details available at https://www.typescriptlang.org/docs/handbook/advanced-types.html#predefined-conditional-types)
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