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Programming TypeScript
book

Programming TypeScript

by Boris Cherny
May 2019
Intermediate to advanced content levelIntermediate to advanced
322 pages
7h 6m
English
O'Reilly Media, Inc.
Book available
Content preview from Programming TypeScript

Appendix C. Scoped Declarations

TypeScript declarations have a rich set of behaviors needed to model types and values, and as in JavaScript, they can be overloaded in a variety of ways. This appendix covers two of these behaviors, summarizing which declarations generate types (and which generate values), and which declarations can be merged.

Does It Generate a Type?

Some TypeScript declarations create a type, some create a value, and some create both. See to Table C-1 for a quick reference.

Table C-1. Does the declaration generate a type?
Keyword Generates a type? Generates a value?

class

Yes

Yes

const, let, var

No

Yes

enum

Yes

Yes

function

No

Yes

interface

Yes

No

namespace

No

Yes

type

Yes

No

Does It Merge?

Declaration merging is a core TypeScript behavior. Take advantage of it to create richer APIs, better modularize your code, and make your code safer.

Table C-2 is reprinted from “Declaration Merging”; it’s a handy reference for which kinds of declarations TypeScript will merge for you.

Table C-2. Can the declaration be merged?
To

Value

Class

Enum

Function

Types alias

Interface

Namespace

Module

Value

No

No

No

No

Yes

Yes

No

 — 

Class

 — 

No

No

No

No

Yes

Yes

 — 

Enum

 — 

 — 

Yes

No

No

No

Yes

 — 

From

Function

 — 

 — 

 — 

No

Yes

Yes

Yes

 — 

Type alias

 — 

 — 

 — 

 — 

No

No

Yes

 — 

Interface

 — 

 — 

 — 

 — 

 — 

Yes

Yes

 — 

Namespace

 — 

 — 

 — 

 — 

 — 

 — 

Yes

 — 

Module

 — 

 — 

 — 

 — 

 — 

 — 

 — 

Yes

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Publisher Resources

ISBN: 9781492037644Errata PageSupplemental Content