cooldown or globalCooldown, not both

[k-tten]

With this interface hierarchy, TypeScript enforces that you can have cooldown or globalCooldown, but not both.

[mcdaddytalk]

One modification, otherwise, the code does not type properly and throws an error:
To union the two Base interfaces to the ICommand interface it is better to use:

interface BaseCommandWithCooldown extends BaseCommand {
    cooldown?: string;
    globalCooldown?: never;
}

interface BaseCommandWithGlobalCooldown extends BaseCommand {
    cooldown?: never;
    globalCooldown?: string;
}

export type ICommand = BaseCommandWithCooldown | BaseCommandWithGlobalCooldown;