I know there's a better way but I'm trying to implement a simple next theme function.

export type Theme = "light" | "dark";

export const nextTheme: Theme = (current: Theme) => {
  switch (current) {
    case "light": {
      return "dark";
    }
    case "dark": {
      return "light";
    }
  }
};

And the error I'm getting is

Type '(current: Theme) => "light" | "dark"' is not assignable to type 'Theme'.

Type '(current: Theme) => "light" | "dark"' is not assignable to type '"dark"'.ts(2322)

I might be having one of those moments, but why wouldn't this be valid? Would it be better to use enums to keep track of the different themes?

CodePudding user response：

You have your return type in the wrong place, it should be after the parentheses:

export type Theme = "light" | "dark";

export const nextTheme = (current: Theme): Theme => {
  switch (current) {
    case "light": {
      return "dark";
    }
    case "dark": {
      return "light";
    }
  }
};