Writing custom declaration files in Typescript - Time & Space Complexity

Time Complexity: Writing custom declaration files

O(n)

Understanding Time Complexity

When writing custom declaration files in TypeScript, it's important to understand how the compiler processes these files.

We want to know how the time to check types grows as the size of the declarations increases.

Scenario Under Consideration

Analyze the time complexity of the following declaration file snippet.

declare module "my-lib" {
  interface User {
    id: number;
    name: string;
  }

  function getUser(id: number): User;
}

// Imagine many interfaces and functions declared similarly

This code declares types and functions for a module, which TypeScript uses for type checking.

Identify Repeating Operations

• Primary operation: The compiler reads and checks each declaration line by line.
• How many times: Once for each declaration in the file, repeated for all declarations.

How Execution Grows With Input

As the number of declarations grows, the compiler spends more time checking each one.

Input Size (n)	Approx. Operations
10	About 10 checks
100	About 100 checks
1000	About 1000 checks

Pattern observation: The time grows roughly in direct proportion to the number of declarations.

Final Time Complexity

Time Complexity: O(n)

This means the time to process the declarations grows linearly with the number of declarations.

Common Mistake

[X] Wrong: "Adding more declarations won't affect compile time much."

[OK] Correct: Each declaration adds work for the compiler, so more declarations mean more time needed.

Interview Connect

Understanding how declaration files affect compile time helps you write efficient types and anticipate build times in real projects.

Self-Check

"What if we split one large declaration file into many smaller files? How would the time complexity change?"

Practice

1. What is the main purpose of writing a custom declaration file in TypeScript?

easy

A. To tell TypeScript about types in JavaScript or untyped libraries

B. To write JavaScript code that runs faster

C. To convert TypeScript code into JavaScript

D. To create new JavaScript functions

Solution

1. Step 1: Understand the role of declaration files

   Declaration files provide type information about JavaScript code or libraries without types.

2. Step 2: Identify the purpose in TypeScript

   This helps TypeScript check code and improve editor support by knowing types.

3. Final Answer:

   To tell TypeScript about types in JavaScript or untyped libraries -> Option A

4. Quick Check:

   Custom declaration files = type info for JS [OK]

Hint: Declaration files describe types for untyped JS code [OK]

Common Mistakes:

• Thinking declaration files speed up code execution
• Confusing declaration files with code transpilation
• Believing declaration files create new functions

2. Which syntax correctly declares a module named 'my-lib' in a custom declaration file?

easy

A. module declare 'my-lib' { function foo(): void; }

B. declare module 'my-lib' { export function foo(): void; }

C. declare 'my-lib' module { export function foo(): void; }

D. module 'my-lib' declare { export function foo(): void; }

Solution

1. Step 1: Recall correct module declaration syntax

   TypeScript uses declare module 'name' { ... } to declare modules.

2. Step 2: Check each option's syntax

   Only declare module 'my-lib' { export function foo(): void; } uses the correct order and keywords.

3. Final Answer:

   declare module 'my-lib' { export function foo(): void; } -> Option B

4. Quick Check:

   Correct module syntax = declare module 'name' [OK]

Hint: Use 'declare module' followed by module name in quotes [OK]

Common Mistakes:

• Swapping 'declare' and 'module' keywords
• Omitting quotes around module name
• Using incorrect keyword order

3. Given this declaration file snippet:

declare module 'utils' {
  export function greet(name: string): string;
}

What is the correct TypeScript output for greet('Alice') if implemented as return `Hello, ${name}!`;?

medium

A. "Hello, Alice!"

B. Error: greet is not defined

C. greet(Alice)

D. Hello, Alice!

Solution

1. Step 1: Understand the declaration and implementation

   The declaration says greet takes a string and returns a string. The implementation returns a template string greeting.

2. Step 2: Evaluate the output of greet('Alice')

   Calling greet('Alice') returns the string Hello, Alice! without quotes in the output.

3. Final Answer:

   Hello, Alice! -> Option D

4. Quick Check:

   Function returns greeting string = Hello, Alice! [OK]

Hint: String return does not include quotes in output display [OK]

Common Mistakes:

• Including quotes around string output incorrectly
• Confusing function call with output text
• Assuming runtime error without implementation

4. Identify the error in this custom declaration file snippet:

declare module 'math-lib' {
  export function add(a: number, b: number): number
}

medium

A. Missing semicolon after function declaration

B. Wrong module name syntax

C. Function parameters must be optional

D. Cannot export functions in declaration files

Solution

1. Step 1: Check syntax inside module declaration

   Each statement inside declaration files must end with a semicolon.

2. Step 2: Identify missing semicolon

   The function declaration line lacks a semicolon at the end.

3. Final Answer:

   Missing semicolon after function declaration -> Option A

4. Quick Check:

   Declaration statements need semicolons [OK]

Hint: Always end declarations with semicolons [OK]

Common Mistakes:

• Forgetting semicolons in declaration files
• Thinking exports are not allowed
• Misnaming modules with wrong quotes

5. You want to add a global type for a new property window.myAppVersion in a TypeScript project. Which custom declaration file snippet correctly adds this global property?

hard

A. declare module 'global' { interface Window { myAppVersion: string; } }

B. declare module 'window' { export const myAppVersion: string; }

C. declare global { interface Window { myAppVersion: string; } } export {};

D. interface Window { myAppVersion: string; }

Solution

1. Step 1: Understand how to add global types

   Use declare global block to augment global interfaces like Window.

2. Step 2: Check each option for correct syntax

   declare global { interface Window { myAppVersion: string; } } export {}; correctly declares global and extends Window interface, then exports empty to make it a module.

3. Final Answer:

   declare global { interface Window { myAppVersion: string; } } export {}; -> Option C

4. Quick Check:

   Global augmentation uses declare global + interface extension [OK]

Hint: Use 'declare global' to add global types safely [OK]

Common Mistakes:

• Trying to declare global types inside a module
• Omitting export {} to make file a module
• Declaring interface without 'declare global'