Spreading possibly undefined variable results in empty object type union

[ajafff]

TypeScript Version: 2.5.1
Code

function foo(a: {[key: string]: string}, b?: {[key: string]: string}) {
    a = {...b, ...a};
}

Expected behavior:
No Error

Actual behavior:

Type '{} | { [x: string]: string; }' is not assignable to type '{ [key: string]: string; }'.
  Type '{}' is not assignable to type '{ [key: string]: string; }'.
    Index signature is missing in type '{}'.

[kitsonk]

Just to note it works without error in playground. Feels like some sort of regression.

[ajafff]

I tested 2.4.2, 2.5.0 and 2.5.1. All of them produce this error.
So this must be a regression somewhere between 2.4.0 and 2.4.2

[jambonrose]

I'm new to TypeScript, but enjoying it immensely - Thank you!

I believe I've just run into the same problem with typescript 2.5.2 .

export interface IFormAction {
  type: FormActions // an enum not shown here
}
export interface IFormState {
  [formName: string]: {
    [fieldName: string]: string
  }
}
export interface IFormReducer {
  (state?: IFormState, action?: IFormAction): IFormState
}
export const reducer: IFormReducer = (
  state, action = { type: FormActions.none }
) => {
  const defaultState: IFormState = {}
  return {
    ...defaultState,
    ...state,
  }
}

src/forms/reducer.ts(42,14): error TS2322: Type '(state: IFormState | undefined, action?: IFormAction | undefined) => {} | { [x: string]: { [field...' is not assignable to type 'IFormReducer'.
  Type '{} | { [x: string]: { [fieldName: string]: string; }; }' is not assignable to type 'IFormState'.
    Type '{}' is not assignable to type 'IFormState'.
      Index signature is missing in type '{}'.

As noted by @ajafff , this seems to be related to object spreads, as removing them removes the error.

export const reducer: IFormReducer = (
  state, action = { type: FormActions.none }
) => {
  const defaultState: IFormState = {}
  return defaultState // compiles no problem
}

Note that explicitly adding | {} onto the relevant type appears to provide a workaround.

export interface IFormReducer {
  (state?: IFormState, action?: IFormAction): IFormState | {}
}
// reducer uses spread as before, but compiles without issue

Hope this is helpful.

[jambonrose]

I believe that #16694 and #17849 reference the same issue.

[mhegazy]

Duplicate of #16694