I’m working with TypeScript 5, and I’m having trouble with type narrowing when using a boolean flag to determine the shape of an object. I expected TypeScript to be able to correctly infer the types, but I’m getting a type error.

Here’s a simplified version of my code:

type Input = {
  isMulti: true,
  value: string[]
} | {
  isMulti: false,
  value: string
}

function multi({ isMulti, value }: Input) {
  if (isMulti) {
    return value[0];
  }
  return value;
}

function test(incomingMulti: boolean) {
  const value = incomingMulti ? ['1'] : '1';
  return multi({ isMulti: incomingMulti, value });
}

Playground

I’m getting the following error on the line where I call the multi function:

Argument of type '{ isMulti: boolean; value: string | string[]; }' is not assignable to parameter of type 'Input'.
  Type '{ isMulti: boolean; value: string | string[]; }' is not assignable to type '{ isMulti: false; value: string; }'.
    Types of property 'isMulti' are incompatible.
      Type 'boolean' is not assignable to type 'false'.

I assumed that TypeScript would be able to infer the correct type based on the value of isMulti:

• if true: value is string[]
• if false: value is string
• if boolean: value is string | string[]

However, it seems to be arbitrarily choosing the false path.

Interestingly, this works:

function test2(incomingMulti: boolean) {
  if (incomingMulti) {
    multi({ isMulti: true, value: ['1'] });
  } else
    multi({ isMulti: false, value: '1' });
}

but I don’t want to force the developers to write it like that every time.