Differences between Interfaces and Type Aliases

Type aliases and interfaces are very similar, and in many cases you can choose between them freely.

Almost all features of an interface are available in type, the key distinction is that a type cannot be re-opened to add new properties vs an interface which is always extendable.

Type:

• Easy way to refer to the different properties + functions that a value has

Primitive Types

• number
• boolean
• void
• undefined
• string
• symbol
• null

  Object Types

• functions
• arrays
• classes
• objects

Interface:

• Extending an interface

  interface Animal {
      name:string
  }
      
  interface Bear extends Animal {
      honey: boolean
  }
      
  const bear = getBear()
  bear.name
  bear.honey
  

Extending a type via intersections

    type Animal = {
        name:string
    }
    
    type Bear = Animal & {
        honey: boolean
    }
    
    const bear = getBear()
    bear.name
    bear.honey

Adding new fields to an existing interface

    interface Window {
	title: string
}
  interface Window {
	ts: TypeScriptAPI
  }

A type cannot be changed after being created

  type Window = {
	title:string
}

  type Window = {
	ts: TypeScriptAPI
  }
// Error: Duplicate identifier 'Window'.