Control Flow Analysis

Definition:

At each instruction, a variable's type may be narrowed according to the current context.

function cfaSample(x: number|string) {
  console.log(x)  // : number|string
  if (typeof x === 'string') {
    console.log(x) // : string
    return x
  }
  return [x] // [number]
} // inferred return type: string|[number]

• Some expressions (typeof x === 'string') act as "type guards", narrowing the possible types of a variable inside a context (the if statement);

• x is narrowed from number|string to string inside the if block;

• x only can by number at the last line, since the if block returns;

• The function gets an inferred return type corresponding to an union of all return paths;

Discriminated union

• The type Actions below is called a discriminated union . The property type is used as a tag to filter out which of the union options is valid at the context;

• At each case line below, action.data has its type narrowed down;

type Actions =
  | { type: "create"; data: { name: string } }
  | { type: "delete"; data: { id: number } }
  | { type: "read"; data: number }

function reducer(action: Actions) {
  switch(action.type) {
    case 'create':
      return createFoo(action.data) // data: {name: string}
    case 'delete':
      return deleteFoo(action.data) // data: {id: number}
    case 'read':
      return readFoo(action.data)   // data: number
  }
}