Migrating JavaScript to TypeScript

Introduction

TypeScript helps catch errors early by adding types to JavaScript. Migrating makes your code safer and easier to understand.

You want to find bugs before running your code.

You want better code suggestions in your editor.

You want to work on a large project with many developers.

You want to use modern JavaScript features with type safety.

You want to improve code readability and maintenance.

Syntax

Typescript

Change file extension from .js to .ts
Add type annotations like:
function greet(name: string): string {
  return `Hello, ${name}!`;
}

TypeScript files use the .ts extension instead of .js.

Adding types like string or number helps the compiler check your code.

Examples

This function adds two numbers and tells TypeScript the inputs and output are numbers.

Typescript

function add(a: number, b: number): number {
  return a + b;
}

Here, we tell TypeScript that message must always be a string.

Typescript

let message: string = 'Hello!';

This declares an array of numbers, so TypeScript knows what type of items it holds.

Typescript

const numbers: number[] = [1, 2, 3];

Sample Program

This program greets a user by name. TypeScript checks that name is a string.

Typescript

function greet(name: string): string {
  return `Hello, ${name}!`;
}

const userName = 'Alice';
console.log(greet(userName));

Important Notes

Start by renaming your .js files to .ts.

Add types gradually; you don't need to type everything at once.

Use any type temporarily if you are unsure, but try to replace it later.

Summary

TypeScript adds types to JavaScript for safer code.

Rename files to .ts and add type annotations step-by-step.

This helps catch errors early and improves code clarity.

Practice

1. What is the main benefit of migrating JavaScript code to TypeScript?

easy

A. It adds type safety to catch errors early

B. It makes the code run faster in browsers

C. It removes all JavaScript features

D. It automatically converts code to Python

Solution

1. Step 1: Understand TypeScript's purpose

   TypeScript adds static types to JavaScript, helping find errors before running code.

2. Step 2: Compare options

   Only It adds type safety to catch errors early correctly states the benefit of type safety. Other options are incorrect or unrelated.

3. Final Answer:

   It adds type safety to catch errors early -> Option A

4. Quick Check:

   Type safety = D [OK]

Hint: TypeScript adds types to catch errors early [OK]

Common Mistakes:

• Thinking TypeScript makes code run faster
• Believing TypeScript removes JavaScript features
• Confusing TypeScript with other languages

2. Which of the following is the correct way to declare a typed variable in TypeScript?

easy

A. let age = number 30;

B. let age: number = 30;

C. let age = 30: number;

D. let age number = 30;

Solution

1. Step 1: Recall TypeScript variable syntax

   TypeScript uses let variableName: type = value; to declare typed variables.

2. Step 2: Check each option

   Only let age: number = 30; follows the correct syntax. Others have misplaced type or incorrect order.

3. Final Answer:

   let age: number = 30; -> Option B

4. Quick Check:

   Correct typed variable syntax = B [OK]

Hint: Use colon after variable name for type [OK]

Common Mistakes:

• Placing type after value
• Omitting colon between name and type
• Using wrong order of type and value

3. What will be the output of this TypeScript code after migration?

function greet(name: string) {
  return `Hello, ${name.toUpperCase()}!`;
}
console.log(greet("world"));

medium

A. TypeError at runtime

B. "Hello, world!"

C. "Hello, WORLD!"

D. SyntaxError during compilation

Solution

1. Step 1: Analyze the function behavior

   The function takes a string and returns a greeting with the name in uppercase.

2. Step 2: Evaluate the output of greet("world")

   Calling greet("world") returns "Hello, WORLD!" because toUpperCase() converts to uppercase.

3. Final Answer:

   "Hello, WORLD!" -> Option C

4. Quick Check:

   String uppercase greeting = C [OK]

Hint: Check string methods and template literals output [OK]

Common Mistakes:

• Ignoring toUpperCase effect
• Expecting original case output
• Confusing runtime and compile errors

4. Identify the error in this TypeScript migration:

function add(a, b: number): number {
  return a + b;
}

medium

A. Parameter 'a' is missing a type annotation

B. Return type 'number' is invalid

C. Function name 'add' is not allowed

D. Parameters must not have types in TypeScript

Solution

1. Step 1: Check parameter types

   In TypeScript, all parameters should have type annotations for clarity and safety.

2. Step 2: Identify missing type

   Parameter 'a' lacks a type, causing a compile error. Other parts are correct.

3. Final Answer:

   Parameter 'a' is missing a type annotation -> Option A

4. Quick Check:

   All parameters need types = A [OK]

Hint: All function parameters need types in TypeScript [OK]

Common Mistakes:

• Leaving some parameters untyped
• Misunderstanding return type syntax
• Thinking function names have restrictions

5. You have this JavaScript object:

const user = { id: 1, name: "Alice", active: true };

How would you migrate it to TypeScript with a proper type annotation?

hard

A. const user: any = { id: 1, name: "Alice", active: true };

B. const user = { id: number, name: string, active: boolean } = { id: 1, name: "Alice", active: true };

C. const user: object = { id: 1, name: "Alice", active: true };

D. const user: { id: number; name: string; active: boolean } = { id: 1, name: "Alice", active: true };

Solution

1. Step 1: Define precise object type

   Use an inline type annotation with exact property types for clarity and safety.

2. Step 2: Compare options

   const user: { id: number; name: string; active: boolean } = { id: 1, name: "Alice", active: true }; correctly annotates the object with property types. const user = { id: number, name: string, active: boolean } = { id: 1, name: "Alice", active: true }; is invalid syntax. const user: object = { id: 1, name: "Alice", active: true }; is too generic. const user: any = { id: 1, name: "Alice", active: true }; disables type checking.

3. Final Answer:

   const user: { id: number; name: string; active: boolean } = { id: 1, name: "Alice", active: true }; -> Option D

4. Quick Check:

   Precise object type annotation = A [OK]

Hint: Use inline type with property names and types [OK]

Common Mistakes:

• Using 'any' which disables type safety
• Incorrect syntax for object type annotation
• Using generic 'object' type losing property info