Flags may not return an undefined value, as undefined can not be serialized.

Specifically, the decide function may not return undefined unless a defaultValue is provided.

When a decide function returns undefined and no defaultValue was provided the flag will throw:

const syncFlag = flag({
  key: '....',
  decide: () => undefined,
});

@vercel/flags: Flag "xxx" must have a defaultValue or a decide function that returns a value

When a decide function returns undefined and a defaultValue was provided the defaultValue will be used:

const syncFlag = flag({
  key: '....',
  decide: () => undefined,
  defaultValue: false
});

By default the decide function's types will not warn when you return undefined as you might have a defaultValue in place. Specify the flag type using flag<boolean> etc to ensure your decide function may not return undefined on a typescript level.

const syncFlag = flag<boolean>({
  key: '....',
  decide: () => undefined, // a typescript warning will now be shown, as we specified the boolean flag type
});