Declaring functions and classes in Typescript - Step-by-Step Execution

Concept Flow - Declaring functions and classes

Start

↓

Declare Function

↓

Call Function?

No→End

Yes↓

Execute Function Body

↓

Return Value

↓

Declare Class

↓

Create Object

↓

Use Object Methods/Properties

↓

End

This flow shows declaring a function, optionally calling it, then declaring a class, creating an object, and using its methods or properties.

Execution Sample

Typescript

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

class Person {
  constructor(public name: string) {}
  greet() { return `Hi, I am ${this.name}`; }
}

Declares a function greet that returns a greeting string, and a class Person with a name property and a greet method.

Execution Table

Step	Action	Evaluation	Result
1	Declare function greet	Function greet(name) created	Function stored in memory
2	Declare class Person	Class Person created with constructor and greet method	Class stored in memory
3	Call greet('Alice')	Execute greet with name='Alice'	Returns 'Hello, Alice!'
4	Create new Person('Bob')	Call constructor with name='Bob'	Object with name='Bob' created
5	Call person.greet()	Execute greet method on Person object	Returns 'Hi, I am Bob'
6	End	No more actions	Program ends

💡 All declarations done and function/class usage demonstrated, program ends.

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
greet	undefined	function greet	function greet	function greet	function greet
Person	undefined	class Person	class Person	class Person	class Person
person	undefined	undefined	Person object {name:'Bob'}	Person object {name:'Bob'}	Person object {name:'Bob'}
greet('Alice') result	undefined	'Hello, Alice!'	'Hello, Alice!'	'Hello, Alice!'	'Hello, Alice!'
person.greet() result	undefined	undefined	undefined	'Hi, I am Bob'	'Hi, I am Bob'

Key Moments - 3 Insights

Why do we declare the function before calling it?

Because in the execution_table at step 1, the function greet is created and stored in memory, so it can be called later at step 3. If we call it before declaration, it would cause an error.

What happens when we create a new Person object?

At step 4, the constructor runs with the given name, creating an object with that name property. This is shown in the variable_tracker where person changes from undefined to an object.

How does the greet method access the name property?

In step 5, person.greet() uses 'this.name' to access the object's name property, returning a string with the name. This shows how methods use 'this' to refer to the current object.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result of calling greet('Alice') at step 3?

A 'Hi, I am Alice'

B undefined

C 'Hello, Alice!'

D Error

💡 Hint

Check the 'Result' column at step 3 in the execution_table.

At which step is the Person object created?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Action' column for object creation in the execution_table.

If we change the name in new Person('Bob') to 'Eve', what changes in variable_tracker after step 4?

A person becomes undefined

B person becomes {name: 'Eve'}

C greet function changes

D No change

💡 Hint

Check the 'person' variable values in variable_tracker after step 4.

Concept Snapshot

Declaring functions:
function name(params): returnType { body }

Declaring classes:
class Name { constructor(...) {...} methods... }

Functions run when called.
Classes create objects with properties and methods.
Use 'this' inside class methods to access object data.

Full Transcript

This visual execution shows how to declare functions and classes in TypeScript. First, the function greet is declared and stored in memory. Then the class Person is declared with a constructor and a greet method. When greet('Alice') is called, it returns a greeting string. Creating a new Person object runs the constructor to set the name property. Calling person.greet() returns a greeting using the object's name. Variables change as functions and objects are created and used. This step-by-step trace helps understand how declarations and calls work in TypeScript.

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)