Declaration merging for namespaces in Typescript

Introduction

Declaration merging lets you combine multiple pieces of code with the same name into one. For namespaces, this means you can add more things to a namespace in different places.

You want to organize related functions or variables under the same namespace but in different files.

You need to extend a namespace with new features without changing the original code.

You want to split a big namespace into smaller parts for easier maintenance.

You want to add helper functions to a namespace after it was first created.

Syntax

Typescript

namespace MyNamespace {
  // first part
}

namespace MyNamespace {
  // second part
}

Both namespace blocks must have the same name to merge.

All merged parts combine into one namespace when the program runs.

Examples

This example shows two parts of the same namespace Animals. Both functions become part of one namespace.

Typescript

namespace Animals {
  export function dog() {
    console.log('Woof!');
  }
}

namespace Animals {
  export function cat() {
    console.log('Meow!');
  }
}

Here, two constants are added to the Tools namespace from separate declarations.

Typescript

namespace Tools {
  export const hammer = 'Hammer';
}

namespace Tools {
  export const screwdriver = 'Screwdriver';
}

Sample Program

This program shows how two namespace declarations with the same name merge. Both hello and goodbye functions are available under Greetings.

Typescript

namespace Greetings {
  export function hello() {
    console.log('Hello!');
  }
}

namespace Greetings {
  export function goodbye() {
    console.log('Goodbye!');
  }
}

Greetings.hello();
Greetings.goodbye();

Important Notes

Declaration merging only works if the namespace names are exactly the same.

Use export inside namespaces to make functions or variables accessible outside.

Declaration merging helps keep code organized and easy to extend.

Summary

Declaration merging combines multiple namespace blocks with the same name into one.

This allows adding new members to a namespace in different places.

It helps organize and extend code without changing original namespace code.

Practice

1. What does declaration merging for namespaces in TypeScript allow you to do?

easy

A. Merge classes and interfaces automatically

B. Combine multiple namespace blocks with the same name into one namespace

C. Create multiple namespaces with different names in one file

D. Prevent namespaces from being extended

Solution

1. Step 1: Understand declaration merging concept

   Declaration merging means combining multiple declarations of the same name into one.

2. Step 2: Apply to namespaces

   For namespaces, this means multiple blocks with the same namespace name merge into a single namespace.

3. Final Answer:

   Combine multiple namespace blocks with the same name into one namespace -> Option B

4. Quick Check:

   Declaration merging = Combine namespaces [OK]

Hint: Namespaces with same name merge into one combined namespace [OK]

Common Mistakes:

• Thinking namespaces with different names merge
• Confusing merging with class or interface merging
• Believing merging prevents extension

2. Which of the following is the correct syntax to merge two namespace declarations named Utils?

easy

A. namespace Utils { export function a() {} } namespace Utils { export function b() {} }

B. namespace Utils { export function a() {} } namespace Utils2 { export function b() {} }

C. namespace Utils { export function a() {} } class Utils { }

D. namespace Utils { export function a() {} } interface Utils { }

Solution

1. Step 1: Check namespace names

   Only namespaces with the exact same name merge. namespace Utils { export function a() {} } namespace Utils { export function b() {} } uses "Utils" twice.

2. Step 2: Verify syntax correctness

   namespace Utils { export function a() {} } namespace Utils { export function b() {} } correctly declares two namespace blocks with functions inside, which will merge.

3. Final Answer:

   namespace Utils { export function a() {} } namespace Utils { export function b() {} } -> Option A

4. Quick Check:

   Same namespace name + multiple blocks = merge [OK]

Hint: Use identical namespace names to merge declarations [OK]

Common Mistakes:

• Using different namespace names expecting merge
• Mixing namespace with class or interface of same name
• Incorrect syntax for namespace blocks

3. What will be the output of the following TypeScript code?

namespace Data {
  export const value = 10;
}
namespace Data {
  export function getValue() { return value; }
}
console.log(Data.getValue());

medium

A. 10

B. undefined

C. ReferenceError

D. SyntaxError

Solution

1. Step 1: Understand namespace merging

   Both namespace blocks named "Data" merge into one namespace containing both value and getValue.

2. Step 2: Evaluate function call

   getValue() returns value which is 10, so console.log prints 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Merged namespace members accessible = 10 [OK]

Hint: Merged namespaces share members, so functions access variables [OK]

Common Mistakes:

• Assuming separate namespaces, so value is undefined
• Expecting runtime errors due to merging
• Confusing variable scope inside merged namespaces

4. Identify the error in the following code that attempts to merge namespaces:

namespace Tools {
  export function toolA() { return 'A'; }
}
namespace Tools {
  const secret = 'hidden';
  export function toolB() { return secret; }
}

medium

A. Functions cannot be declared in merged namespaces

B. Duplicate namespace name causes syntax error

C. Cannot access non-exported 'secret' in merged namespace

D. 'secret' must be exported to be used inside toolB

Solution

1. Step 1: Check variable scope in merged namespaces

   The variable secret is declared but not exported, so it is private to its namespace block.

2. Step 2: Analyze usage of 'secret' in toolB

   Function toolB tries to return secret, but since secret is not accessible outside its block, this causes an error.

3. Final Answer:

   Cannot access non-exported 'secret' in merged namespace -> Option C

4. Quick Check:

   Non-exported members are block-scoped [OK]

Hint: Non-exported members are private to their block, not merged [OK]

Common Mistakes:

• Assuming all members merge regardless of export
• Thinking duplicate namespace names cause syntax errors
• Believing functions can't be declared in merged namespaces

5. Given two namespace declarations below, what will console.log(Shapes.area(5)); output?

namespace Shapes {
  export function area(radius: number) {
    return Math.PI * radius * radius;
  }
}
namespace Shapes {
  export function perimeter(radius: number) {
    return 2 * Math.PI * radius;
  }
  export function area(radius: number) {
    return Math.round(Math.PI * radius * radius);
  }
}
console.log(Shapes.area(5));

hard

A. 78.53981633974483

B. Error: Duplicate function name

C. NaN

D. 79

Solution

1. Step 1: Understand merging with duplicate function names

   When namespaces merge, later declarations override earlier ones for the same member name.

2. Step 2: Identify which 'area' function is used

   The second namespace redefines area with rounding. So Shapes.area(5) calls the rounded version.

3. Step 3: Calculate output

   Math.PI * 5 * 5 ≈ 78.5398, rounded is 79.

4. Final Answer:

   79 -> Option D

5. Quick Check:

   Later declarations override earlier ones [OK]

Hint: Later namespace members override earlier ones with same name [OK]

Common Mistakes:

• Expecting both functions to coexist without override
• Thinking duplicate function names cause errors
• Ignoring rounding in second area function