Constructor parameter types in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript class constructor?

Consider this TypeScript class with typed constructor parameters. What will be the output when creating an instance and logging its properties?

Typescript

class Person {
  name: string;
  age: number;
  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}
const p = new Person('Alice', 30);
console.log(`${p.name} is ${p.age} years old.`);

A Alice is 30 years old.

B undefined is undefined years old.

C Error: Property 'name' does not exist on type 'Person'.

D Alice is undefined years old.

💡 Hint

Check how the constructor assigns the parameters to the class properties.

The constructor correctly assigns the name and age parameters to the class properties. So logging them outputs the expected string.

❓ Predict Output

intermediate

What error occurs with this constructor parameter type mismatch?

What error will TypeScript show for this class when trying to create an instance?

Typescript

class Car {
  model: string;
  year: number;
  constructor(model: string, year: number) {
    this.model = model;
    this.year = year;
  }
}
const c = new Car('Tesla', '2020');

A Property 'year' is missing in type 'string'.

B No error, instance created successfully.

C Type 'string' is not assignable to type 'number'.

D SyntaxError: Unexpected string.

💡 Hint

Look at the type of the second argument passed to the constructor.

The constructor expects year to be a number, but a string is passed. TypeScript reports a type mismatch error.

🔧 Debug

advanced

Why does this constructor cause a runtime error?

Examine the class and its constructor. Why does this code cause a runtime error?

Typescript

class Rectangle {
  width: number;
  height: number;
  constructor(width: number, height: number) {
    this.width = width;
    // Forgot to assign height
  }
  area() {
    return this.width * this.height;
  }
}
const r = new Rectangle(5, 10);
console.log(r.area());

A Output: NaN

B Output: 50

C Runtime error: Cannot read property 'height' of undefined.

D SyntaxError: Missing assignment in constructor.

💡 Hint

Check if all properties are initialized before use.

The height property is never assigned a value, so it is undefined. Multiplying a number by undefined results in NaN.

📝 Syntax

advanced

Which constructor syntax is correct for parameter properties?

Which option correctly uses TypeScript's parameter properties to declare and initialize class members in the constructor?

A

class Point {
  constructor(x: number, y: number) {
    this.x: number = x;
    this.y: number = y;
  }
}

B

class Point {
  constructor(x: number, y: number) {
    public this.x = x;
    private this.y = y;
  }
}

C

class Point {
  constructor(public x: number, y: number) {
    this.y = y;
  }
}

D

class Point {
  constructor(public x: number, private y: number) {}
}

💡 Hint

Parameter properties combine declaration and assignment in the constructor parameters.

Option D correctly declares x and y as class members with access modifiers in the constructor parameters. Other options have syntax errors or misuse.

🚀 Application

expert

How many properties does this class instance have after construction?

Given this TypeScript class using parameter properties and explicit members, how many own properties will an instance have?

Typescript

class User {
  id: number;
  constructor(public name: string, id: number) {
    this.id = id;
  }
}
const u = new User('Bob', 42);
console.log(Object.keys(u).length);

A 1

B 2

C 0

D 3

💡 Hint

Parameter properties create class members that are own properties. Explicit assignments also create own properties.

The instance u has two own properties: name (from parameter property) and id (assigned in constructor). So Object.keys(u).length is 2.

Practice

1. What does specifying parameter types in a TypeScript class constructor do?

easy

A. It changes the class name based on parameters.

B. It runs the constructor faster at runtime.

C. It automatically creates getter and setter methods.

D. It tells what kind of values the constructor expects when creating an object.

Solution

1. Step 1: Understand constructor parameter types

   Constructor parameter types define what type of values the constructor expects to receive when creating an instance of the class.

2. Step 2: Identify the purpose of parameter types

   They help TypeScript check for errors before running the code by ensuring the right types are passed.

3. Final Answer:

   It tells what kind of values the constructor expects when creating an object. -> Option D

4. Quick Check:

   Constructor parameter types = Define expected input types [OK]

Hint: Constructor types define expected inputs clearly [OK]

Common Mistakes:

• Thinking parameter types speed up runtime
• Confusing parameter types with method creation
• Believing parameter types rename classes

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

easy

A. constructor(name: string, age: number) {}

B. constructor(name string, age number) {}

C. constructor(name, age) : string, number {}

D. constructor(name => string, age => number) {}

Solution

1. Step 1: Review TypeScript constructor syntax

   In TypeScript, parameter types are declared after the parameter name with a colon, like name: string.

2. Step 2: Check each option

   constructor(name: string, age: number) {} uses correct syntax with colons and types. Options A, C, and D use invalid syntax.

3. Final Answer:

   constructor(name: string, age: number) {} -> Option A

4. Quick Check:

   Parameter types use colon syntax [OK]

Hint: Use colon after parameter name for types [OK]

Common Mistakes:

• Omitting colons between name and type
• Using commas or arrows instead of colons
• Placing types after parentheses

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

class Person {
  constructor(public name: string, private age: number) {}
  getAge() { return this.age; }
}
const p = new Person('Alice', 30);
console.log(p.name);
console.log(p.getAge());

medium

A. Error: age is private

B. Alice
30

C. Alice
undefined

D. undefined
30

Solution

1. Step 1: Understand public and private in constructor

   The public name creates a public property accessible outside. The private age creates a private property accessible only inside the class.

2. Step 2: Check console.log outputs

   p.name prints 'Alice' because it's public. p.getAge() returns 30 because the method accesses private age internally.

3. Final Answer:

   Alice 30 -> Option B

4. Quick Check:

   Public accessible, private via method [OK]

Hint: Public props accessible directly, private via methods [OK]

Common Mistakes:

• Expecting private properties accessible outside
• Confusing undefined with private access
• Thinking private causes runtime error

4. Identify the error in this TypeScript class constructor:

class Car {
  constructor(make: string, year: number) {
    this.make = make;
    this.year = year;
  }
}

medium

A. Constructor must have return type declared

B. Cannot assign parameters to 'this' inside constructor

C. Class properties 'make' and 'year' are missing

D. Missing type for parameter 'year' in constructor

Solution

1. Step 1: Check class properties and constructor assignments

2. Step 2: Check class properties

   The class does not declare properties 'make' and 'year' before assigning them in the constructor, which causes an error: Property 'make' does not exist on type 'Car'.

3. Final Answer:

   Class properties 'make' and 'year' are missing -> Option C

4. Quick Check:

   Declare class properties before use [OK]

Hint: Declare properties before assigning in constructor [OK]

Common Mistakes:

• Ignoring missing property declarations
• Thinking constructor parameters auto-create properties
• Expecting return type in constructor

5. You want to create a class Book that takes a title (string), pages (number), and an optional author (string). Which constructor declaration correctly types these parameters and allows creating a Book by omitting the author argument (leaving author as undefined)?

hard

A. constructor(title: string, pages: number, author?: string) {}

B. constructor(title: string, pages: number, author: string = '') {}

C. constructor(title: string, pages: number, author: string) {}

D. constructor(title: string, pages: number, author?: string = '') {}

Solution

1. Step 1: Understand optional parameters in TypeScript

   Optional parameters are marked with a question mark ? after the parameter name, allowing them to be omitted when calling the constructor, leaving the parameter undefined.

2. Step 2: Evaluate each option

   constructor(title: string, pages: number, author?: string = '') {}: author?: string = '' -- omitting sets to '', not undefined. constructor(title: string, pages: number, author: string = '') {}: author: string = '' -- omitting sets to ''. constructor(title: string, pages: number, author: string) {} requires author. constructor(title: string, pages: number, author?: string) {}: author?: string -- omitting leaves undefined.

3. Final Answer:

   constructor(title: string, pages: number, author?: string) {} -> Option A

4. Quick Check:

   Optional params use ? after name [OK]

Hint: Use ? after parameter name for optional [OK]

Common Mistakes:

• Using default value without ? for optional
• Forgetting to mark optional parameters
• Mixing optional and default syntax incorrectly