1. Control Flow Analysis

If Statements

Most narrowing comes from expressions inside if statements, where different type operators narrow inside the new scope

typeof (for primitives)

const input = getUserInput();

// input string | number
if (typeof input === 'string') {
  // input string
}

instanceof (for classes)

const input = getUserInput();

// input string | number[]
if (instance of Array) {
  // input number[]
}

"property" in object (for object)

const input = getUserInput();

// input string | { error: ...}
if ('error' in input) {
  // input { error: ...}
}