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Coggle requires JavaScript to display documents.

15.1: Type narrowing - Coggle Diagram

- - - - could be undefined
      - type is string | undefined
    - - coerces the value a truthy value
        
        in this case, string
        
        it narrows the type
      - so within the for loop
        
        the type is string
    - - :!!: type is still string | undefined
      - truthy check in if
        
        doesn't guarantee that word
        
        is not a string
        
        it's just not truthy
        
        it could be an empty string
- - - - type Admin = { name: string privileges: string[] } type Employee = { name: string startDate: Date } type ElevatedEmployee = Admin & Employee type UnknownEmployee = Employee | Admin
      - function printEmployeeInformation(emp: UnknownEmployee) { console.log('Name: ' + emp.name); if ('privileges' in emp) { console.log('Privileges: ' + emp.privileges); } if ('startDate' in emp) { console.log('Start Date: ' + emp.startDate); } }
  - - - it's not a type that JavaScript knows
      - and the test will run at runtime
        
        when it's been compiled to JavaScript
  - - - is in the specified object
        
        or its prototype chain