Declaring functions and classes in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you declare a simple function in TypeScript that adds two numbers?

You declare it using the <code>function</code> keyword, specify parameter types, and return type. Example:<br><pre>function add(a: number, b: number): number {<br>  return a + b;<br>}</pre>

beginner

What is the syntax to declare a class with a constructor in TypeScript?

Use the <code>class</code> keyword, define a <code>constructor</code> method to initialize properties. Example:<br><pre>class Person {<br>  name: string;<br>  constructor(name: string) {<br>    this.name = name;<br>  }<br>}</pre>

beginner

What does the <code>this</code> keyword refer to inside a class method?

this refers to the current instance of the class, allowing access to its properties and other methods.

intermediate

How do you declare a function expression with a type annotation in TypeScript?

You can assign a function to a variable with a type. Example:<br><pre>const multiply: (x: number, y: number) => number = (x, y) => x * y;</pre>

intermediate

What is the difference between a function declaration and a function expression in TypeScript?

A function declaration is hoisted and can be called before it is defined. A function expression is assigned to a variable and is not hoisted, so it must be defined before use.

Which keyword is used to declare a class in TypeScript?

A class

B function

C object

D struct

The class keyword is used to declare classes in TypeScript.

What is the correct way to specify the return type of a function that returns a string?

A function greet(): String {}

B function greet(): string {}

C function greet(): str {}

D function greet() -> string {}

In TypeScript, the return type is specified after the parameter list with a colon, like (): string.

How do you access a property named age inside a class method?

A this.age

B self.age

C age

D class.age

Inside a class method, use this.propertyName to access properties.

Which of the following is a valid function expression in TypeScript?

A let f = (x) => x * 2;

B function f(x: number): number { return x * 2; };

C const f = function(x: number): number { return x * 2; };

D var f: function = (x) => x * 2;

Option C shows a valid function expression with type annotations.

What happens if you call a function expression before it is defined?

A It works fine because all functions are hoisted.

B It calls the function but with default parameters.

C It returns undefined.

D It causes an error because function expressions are not hoisted.

Function expressions are not hoisted, so calling them before definition causes an error.

Explain how to declare a class with a constructor and a method in TypeScript.

Think about how you create a blueprint for objects with properties and actions.

Describe the difference between function declarations and function expressions in TypeScript.

Consider when you can call the function in your code relative to where it is written.

Practice

1. What is the main purpose of a function in TypeScript?

easy

A. To create a new type of variable

B. To store data without any behavior

C. To perform a specific task and optionally return a result

D. To define a constant value

Solution

1. Step 1: Understand what a function does

   A function is a block of code designed to perform a specific task.

2. Step 2: Recognize the role of return values

   Functions can return results after performing their task, making them reusable.

3. Final Answer:

   To perform a specific task and optionally return a result -> Option C

4. Quick Check:

   Function purpose = perform task + return result [OK]

Hint: Functions do tasks and can give back results [OK]

Common Mistakes:

• Confusing functions with variables
• Thinking functions only store data
• Believing functions cannot return values

2. Which of the following is the correct way to declare a function named greet that takes a string name and returns a string in TypeScript?

easy

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

B. function greet(name) { return 'Hello, ' + name; }

C. function greet(name: string) { console.log('Hello, ' + name); }

D. function greet(): string { return name; }

Solution

1. Step 1: Check function parameter and return types

   function greet(name: string): string { return `Hello, ${name}`; } correctly declares the parameter name as a string and the return type as string.

2. Step 2: Verify function body returns a string

   function greet(name: string): string { return `Hello, ${name}`; } returns a template string greeting the name, matching the return type.

3. Final Answer:

   function greet(name: string): string { return `Hello, ${name}`; } -> Option A

4. Quick Check:

   Correct parameter and return types = function greet(name: string): string { return `Hello, ${name}`; } [OK]

Hint: Specify parameter and return types explicitly [OK]

Common Mistakes:

• Omitting parameter types in TypeScript
• Not returning a value when return type is declared
• Using console.log instead of return

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

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet() {
    return `Hi, I am ${this.name}`;
  }
}
const p = new Person('Anna');
console.log(p.greet());

medium

A. Hi, I am Anna

B. Hi, I am undefined

C. Error: Property 'name' does not exist

D. Hi, I am

Solution

1. Step 1: Understand class construction

   The Person class sets this.name to the passed argument 'Anna' in the constructor.

2. Step 2: Analyze the greet method output

   The greet method returns a string including this.name, which is 'Anna'.

3. Final Answer:

   Hi, I am Anna -> Option A

4. Quick Check:

   Class property set correctly = 'Hi, I am Anna' [OK]

Hint: Constructor sets properties; methods use them [OK]

Common Mistakes:

• Forgetting to assign constructor parameter to this.name
• Expecting undefined because of missing this
• Confusing method return with console.log output

4. Identify the error in this TypeScript class declaration:

class Car {
  model: string;
  constructor(model) {
    this.model = model;
  }
  display() {
    console.log('Model: ' + this.model);
  }
}

medium

A. Property 'model' should be private

B. Missing return type for display method

C. Constructor should not have parameters

D. Missing type annotation for constructor parameter 'model'

Solution

1. Step 1: Check constructor parameter types

   In TypeScript, constructor parameters should have type annotations; here, model lacks a type.

2. Step 2: Verify other parts

   The display method does not require a return type annotation since it returns void implicitly.

3. Final Answer:

   Missing type annotation for constructor parameter 'model' -> Option D

4. Quick Check:

   Constructor parameters need types = Missing type annotation for constructor parameter 'model' [OK]

Hint: Always type constructor parameters in TypeScript [OK]

Common Mistakes:

• Omitting parameter types in constructors
• Thinking methods must always declare return types
• Assuming properties must be private by default

5. You want to create a TypeScript class Calculator with a method add that takes two numbers and returns their sum. Which code correctly implements this with proper types and usage?

hard

A. class Calculator {
add(a, b) {
return a + b;
}
}
const calc = Calculator();
console.log(calc.add(5, 3));

B. class Calculator {
add(a: number, b: number): number {
return a + b;
}
}
const calc = new Calculator();
console.log(calc.add(5, 3));

C. class Calculator {
add(a: number, b: number) {
console.log(a + b);
}
}
const calc = new Calculator();
calc.add(5, 3);

D. class Calculator {
add(a: number, b: number): number {
return a + b;
}
}
const calc = new Calculator();
console.log(calc.add('5', '3'));

Solution

1. Step 1: Check method declaration and types

   class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); declares add with parameters typed as numbers and return type number, which is correct.

2. Step 2: Verify object creation and method call

   class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); creates an instance with new Calculator() and calls add(5, 3), which returns 8.

3. Final Answer:

   Correct class with typed method and proper usage -> Option B

4. Quick Check:

   Typed method + new instance + correct call = class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); [OK]

Hint: Use 'new' to create class instances and type parameters [OK]

Common Mistakes:

• Calling class without 'new'
• Omitting parameter types
• Passing wrong argument types (like strings instead of numbers)