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Type aliases

Sometimes, you have to reuse a type elsewhere in your code. Instead of repeating the type definition, you can use a type alias.

// You may alias a primitive type
type Name = string
// And even alias an alias
type FullName = Name;
// Aliases are useful to define complex types
type SuperLongType = string | number | boolean | null | undefined;
// ^ Union types are covered in a later chapter

const fullName: FullName = 'Alice Smith';

function printName(name: Name) {
  console.log(`Name: ${name}`);
}

Type aliases vs variable declarations​

Type aliases are similar to JavaScript variable declarations in many ways:

  • They are used to store values for later use
    • Type aliases store types for later use
    • JavaScript variables store values for later use

You can think of type aliases as "variables for types".