Declaration merging is the process where the compiler merges two or more declarations with the same name into a single definition. Declarations can create namespaces, types, or values. Interface merging joins the members of interface declarations with the same name into a single interface, requiring unique members. Namespace merging combines exported functions or classes from namespaces with the same name. Declaration merging also allows merging namespaces with classes or functions by referencing properties and values from the namespace. However, classes cannot merge with other classes or variables in TypeScript.

1. Declaration Merging
JAGADEESH PATTA ( PJ )

2. Introduction
 Declaration Merging is the process followed by the compiler
to merge two or more declarations.
 The declarations declared with same name into a single
definition.

3. Intro…
 A declaration creates entities in at least one of three groups.
Namespace
Type
Value

4. Namespace
 Creating declarations create a namespace, which contains
names that are accessed using a dotted notation.
namespace namespace_name{
declarations
}

5. Types
 Creating declarations create a type that is visible with the
declared shape and bound to given name
Eg:
Classes
Interfaces
Enum, Etc.

6. Values
 Creating declarations create a value that are visible in the
output JavaScript.
Eg:
Classes
Namespace
Function, Variable
Enum, Etc.

7. Interface Merging
 In interface merging, The merge joins the members of both
declarations into a single interface with same name.
 In interface merging the members must be unique.
 The function members of interface is overloaded.

8. Interface Merging…
Interface Box{
sayHai();
}
Interface Box{
sayBai();
}

9. Namespace Merging…
namespace Sample{
export function sayHai(){
console.log(‘hai’);
}
}
namespace Sample{
export function sayBye(){
console.log(‘Bye’);
}
}

10. Merging namespace with class
class Album{
label: Album.AlbumLabel;
}
namespace Album{
export class AlbumLabel { }
}

11. Merging namespace with function
Function buildLabel(name){
return buildLabel.prefix + name +
buildLabel.suffix;
}
namespace buildLabel{
export let suffix = "";
export let prefix = "Hello, ";
}

12. Disallowed Merges
 In Typescript classes can not merge with other classes or with
variables.

13. Thank You