Can you explain the significance of the "type" reserved word in TypeScript?

When attempting to construct an interface in TypeScript, I discovered that the word "type" is both a keyword and a reserved term. In Visual Studio 2013 with TypeScript 1.4, for instance, when defining the following interface:

interface IExampleInterface {
    type: string;
}

the term "type" appears in blue. If you then proceed to implement this interface in a class:

class ExampleClass implements IExampleInterface {
    public type: string;

    constructor() {
        this.type = "Example";
    }
}

Upon typing "type" to fulfill the required property, IntelliSense treats it as a keyword like "typeof" or "new". Searching further, I came across this GitHub issue which categorizes "type" as a "strict mode reserved word" in TypeScript. However, its exact purpose remains unclear.

I may be missing something obvious, but what exactly does the "type" reserved word signify in TypeScript?

typescriptkeywordreserved-words

Answer №1

One common application of this feature is for defining "type aliases". Here's a couple examples:

type EmailAddress = string | null;
type PersonInfo = Map<string, Info>;

If you want to learn more about type aliases in TypeScript, you can check out the relevant section in the official TypeScript Handbook.

Answer №2

Explanation of the type keyword in TypeScript:

The type keyword in TypeScript is used to create an alias for a specific type. It can also be utilized to define user-defined types. To better understand this concept, let's look at an example:

type Age = number | string;    // 'pipe' symbol denotes 'number OR string'
type Color = "blue" | "red" | "yellow" | "purple";
type Random = 1 | 2 | 'random' | boolean;

// Here, type madness can consist of any value within the types defined by random and color,
// along with the numeric value '3' and the string value 'foo'.
type Madness = Random | 3 | 'foo' | Color; 

type ErrorType = Error | null;
type CallbackFunction = (err: ErrorType, res: Color) => Random;

In TypeScript, you can combine scalar types such as string, number, etc., as well as literal values like 1 or 'mystring'. Additionally, you can create composite types using other user-defined types. For instance, the type Madness includes the types Random and Color.

When attempting to assign a string literal to a variable (with IntelliSense enabled), your IDE will provide suggestions:

These suggestions include all colors derived from the type Color, 'random' derived from type Random, and the string 'foo' specified in the type Madness itself.

Answer №3

Using Type Aliases in TypeScript allows you to create custom names for types.

Type Aliases are versatile and can be applied to basic data types like strings, as well as more intricate types such as objects and arrays:

For example:

type CarYear = number
type CarType = string
type CarModel = string
type Car = {
  year: CarYear,
  type: CarType,
  model: CarModel
}

const carYear: CarYear = 2001
const carType: CarType = "Toyota"
const carModel: CarModel = "Corolla"
const car: Car = {
  year: carYear,
  type: carType,
  model: carModel
};

Find out more here!

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