TypeScript: Define Type

By Xah Lee. Date: .

Define Your Own Named Type

you can define your own named type.

there are two ways, one is by defining a type alias and the other way is to define a interface.

Type Alias

type TypeName = TypeSpec

example 

// define a named type. either a number or string
type NumOrStr  = number | string;
// define a named type. a object that represent coordinate of a point
type Point = {
  x: number;
  y: number;
};

Interface

interface TypeName = TypeSpec

example 

// define a named type. a object that represent coordinate of a point
interface Point = {
  x: number;
  y: number;
};

What's the Difference Between “type” and “interface”

basically, a interface type spec can be extended later in a different place.

For beginners, you don't need to worry about them. Use either.

Type Assertions (like “casting” type)

sometimes you need to force the type spec of a value from one to another. (normally, type spec is specified for a variable. here, it's for a value, often the value of a variable.)

two syntax for this:

example 

const xx = ((<HTMLInputElement> searchBox).value)
const xx = ((searchBox as HTMLInputElement).value);

this usually happen with DOM objects. because DOM was not well designed some decade ago before TypeScript.

TypeScript, type sytax