What is the "type" reserved word in TypeScript?

Question

I just noticed when trying to create an interface in TypeScript that "type" is either a keyword or a reserved word. When creating the following interface, for example, "type" is shown in blue in Visual Studio 2013 with TypeScript 1.4:

interface IExampleInterface {
    type: string;
}

Let's say that you then try to implement the interface in a class, like this:

class ExampleClass implements IExampleInterface {
    public type: string;

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

In the first line of the class, as you type (sorry) the word "type" in order to implement the property required by the interface, IntelliSense appears with "type" having the same icon as other keywords like "typeof" or "new".

I've had a look around, and could find this GitHub issue which lists "type" as a "strict mode reserved word" in TypeScript, but I have not found any further information about what its purpose actually is.

I suspect I'm having a brain fart and this is something obvious I should already know, but what is the "type" reserved word in TypeScript for?

`Type Alias` to give your types a semantic name : http://basarat.gitbooks.io/typescript/content/docs/types/type-system.html — basarat, Jul 12 '15 at 09:01

Jcl · Accepted Answer · 2021-04-14T04:20:59.433

182

It's used for "type aliases". For example:

type StringOrNumber = string | number;
type DictionaryOfStringAndPerson = Dictionary<string, Person>;

Reference: (edit: removed outdated link) TypeScript Specification v1.5 (section 3.9, "Type Aliases", pages 46 & 47)

Update: (edit: removed outdated link) Now on section 3.10 of the 1.8 spec. Thanks @RandallFlagg for the updated spec and link

Update: (edit: deprecated link) TypeScript Handbook, search "Type Aliases" can get you to the corresponding section.

Update: Now it's here in the TypeScript Handbook.

edited Apr 14 '21 at 04:20

answered Jul 12 '15 at 05:40

Jcl

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Yep, so it was something obvious. Turns out it's quite hard to find what you're looking for when you search for the word "type" in the context of programming languages – particularly when the language in question is called "TypeScript". By the way, generic dictionaries still don't exist in TypeScript right? – Adam Goodwin Jul 12 '15 at 05:48
1

[Here's one](https://github.com/basarat/typescript-collections) if you need it (for TS 0.9 and above): – Jcl Jul 12 '15 at 05:52
Thanks, I think I saw that project once before when I wanted a dictionary, but in the end I decided just to do without. – Adam Goodwin Jul 12 '15 at 06:07
the concept came from typedef in c a check this https://www.studytonight.com/c/typedef.php – Pranoy Sarkar Jul 15 '18 at 07:55
The first two links are broken. Please update, thanks :) – Serkan Sipahi Sep 16 '20 at 18:39
@SerkanSipahi removed outdated links, although left the text for clarity. Thanks :-) – Jcl Sep 17 '20 at 08:10

score 50 · Answer 2 · edited Jan 04 '20 at 07:25

50

Type keyword in typescript:

In typescript the type keyword defines an alias to a type. We can also use the type keyword to define user defined types. This is best explained via an example:

type Age = number | string;    // pipe means number OR string
type color = "blue" | "red" | "yellow" | "purple";
type random = 1 | 2 | 'random' | boolean;

// random and color refer to user defined types, so type madness can contain anything which
// within these types + the number value 3 and string value 'foo'
type madness = random | 3 | 'foo' | color;  

type error = Error | null;
type callBack = (err: error, res: color) => random;

You can compose types of scalar types (string, number, etc), but also of literal values like 1 or 'mystring'. You can even compose types of other user-defined types. For example type madness has the types random and color in it.

Then when we try to make a string literal our (and we have IntelliSense in our IDE) it shows suggestions:

It shows all the colors, which type madness derives from having type color, 'random' which is derived from type random, and finally, the string 'foo' which is on the type madness itself.

edited Jan 04 '20 at 07:25

Sinandro

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answered Nov 04 '18 at 10:34

Willem van der Veen

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What is the difference between user defined types and enumerables – ORcoder Sep 13 '19 at 00:24
1

@ORcoder good question! I want to know also. A good explanation for us to understand! – Merv Sep 28 '19 at 21:53
where you have declared the `type color = "blue" | "red" | "yellow" | "purple";` statements inside or outside the class ? – Lalit Kushwah Nov 29 '19 at 09:57
[This might help](https://stackoverflow.com/questions/40275832/typescript-has-unions-so-are-enums-redundant) regarding the difference between user defined types and enums – Christophe Fuzier Aug 27 '20 at 12:45
type keyword with values should be declared outside the class. – Prabhakaran M Jan 11 '23 at 10:07

score 0 · Answer 3 · answered May 30 '23 at 02:44

Type Aliases allow defining types with a custom name (an Alias).

Type Aliases can be used for primitives like string or more complex types such as objects and arrays:

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
};

Reference

What is the "type" reserved word in TypeScript?

3 Answers3

Type keyword in typescript:

Linked