TypeScript Access Modifiers |

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Access Modifiers are one of the core features of TypeScript’s object-oriented programming.

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They are used to control the visibility of class members (properties, methods, constructors).

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Through access modifiers, encapsulation can be achieved, protecting the internal implementation details of a class.

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Why Access Modifiers Are Needed

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One of the three fundamental principles of object-oriented programming is encapsulation.

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Encapsulation means hiding data and the methods that operate on that data, exposing only necessary interfaces to the outside.

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Access modifiers are the means to achieve encapsulation, controlling the scope in which class members can be accessed.

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Concept Explanation: Access modifiers determine where class members can be accessed. TypeScript provides three access modifiers: public, protected, and private.
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public Modifier

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public is the default access modifier, indicating that the member can be accessed from anywhere.

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Whether inside the class, in subclasses, or outside the class, public members can be accessed.

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Example

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// Define the Animal class\\n\\nclass Animal {\\n\\n  // Use public modifier for name property (can be omitted, default is public)\\n\\n  public name: string;\\n\\n  // Constructor\\n\\n  public constructor(name: string){\\n\\n    this.name= name;\\n\\n  }\\n\\n  // Public speak method\\n\\n  public speak():void{\\n\\n    console.log(this.name+" make a sound");\\n\\n  }\\n\\n}\\n\\n// Create an instance\\n\\nvar animal =new Animal("Animal");\\n\\n// Access public property from outside the class\\n\\nconsole.log(animal.name);\\n\\n// Call public method from outside the class\\n\\nanimal.speak();\\n

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Output:

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Animal Animal make a sound\\n

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Note: If no access modifier is specified, TypeScript defaults to public. Thus, public can be omitted, but for code clarity, it is recommended to declare it explicitly.
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private Modifier

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private indicates a private member, accessible only within the class where it is defined.

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Neither subclasses nor external code can access private members.

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This is commonly used to hide internal implementation details and protect data from unintended modification.

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Example

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// Define the BankAccount class\\n\\nclass BankAccount {\\n\\n  // Use private modifier for balance; accessible only within the class\\n\\n  private balance: number;\\n\\n  // Constructor\\n\\n  constructor(initialBalance: number){\\n\\n    this.balance= initialBalance;\\n\\n  }\\n\\n  // Deposit method\\n\\n  public deposit(amount: number):void{\\n\\n    if(amount >0){\\n\\n      this.balance+= amount; // Private property accessible within the class\\n\\n      console.log("Deposit successful, current balance: "+this.balance);\\n\\n    }\\n\\n  }\\n\\n  // Get balance\\n\\n  public getBalance(): number {\\n\\n    return this.balance; // Private property accessible within the class\\n\\n  }\\n\\n}\\n\\n// Create an account instance\\n\\nvar account =new BankAccount(1000);\\n\\n// Deposit\\n\\naccount.deposit(500);\\n\\n// Get balance via public method\\n\\nconsole.log("balance: "+ account.getBalance());\\n\\n// Error: Cannot directly access private property from outside the class\\n\\n// console.log(account.balance); // Compilation error!\\n

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Output:

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Deposit successful, current balance: 1500balance: 1500\\n

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Best Practice: Set internal state of a class as private and provide controlled access via public methods—this is the standard approach to encapsulation.
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protected Modifier

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protected indicates a protected member, accessible within the defining class and its subclasses.

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Protected members cannot be accessed directly from outside the class.

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This is useful when subclasses need to inherit certain functionality from the parent class, while hiding implementation details.

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Example

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// Define the base Person class\\n\\nclass Person {\\n\\n  // Use protected modifier for name; accessible in subclasses\\n\\n  protected name: string;\\n\\n  constructor(name: string){\\n\\n    this.name= name;\\n\\n  }\\n\\n}\\n\\n// Define the Employee class, extending Person\\n\\nclass Employee extends Person {\\n\\n  // Department is private\\n\\n  private department: string;\\n\\n  constructor(name: string, department: string){\\n\\n    super(name); // Call parent constructor\\n\\n    this.department= department;\\n\\n  }\\n\\n  // Introduce method\\n\\n  public introduce(): string {\\n\\n    // Subclass can access protected member name\\n\\n    return "I am "+this.name+"，in "+this.department+" work";\\n\\n  }\\n\\n}\\n\\n// Create an employee instance\\n\\nvar emp =new Employee("Alice","Technical Department");\\n\\nconsole.log(emp.introduce());\\n\\n// Error: Cannot access protected member from outside the class\\n\\n// console.log(emp.name); // Compilation error!\\n

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Output:

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I am Alice，in Technical Department work\\n

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Use Case: protected is commonly used for properties that subclasses need to use, but should not be exposed externally.
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readonly Modifier

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readonly is used to make a property read-only.

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It can only be assigned during declaration or within the constructor; after that, it cannot be modified.

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This is useful for defining constants or identifiers.

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Example

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// Define the User class\\n\\nclass User {\\n\\n  // A readonly property can only be assigned during initialization\\n\\n  // User ID\\n\\n  readonly id: number;\\n\\n  // Username\\n\\n  readonly name: string;\\n\\n  constructor(id: number, name: string){\\n\\n    this.id= id;\\n\\n    this.name= name;\\n\\n  }\\n\\n}\\n\\n// Create a user instance\\n\\nvar user =new User(1,"Alice");\\n\\nconsole.log("User: "+ user.id+", "+ user.name);\\n\\n// Error: Cannot modify readonly properties\\n\\n// user.id = 2; // Compilation error!\\n\\n// user.name = "Bob"; // Compilation error!\\n

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Output:

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User: 1, Alice\\n

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Note: readonly does not conflict with private. They can be used together—making the property both unmodifiable and inaccessible from outside.
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Parameter Properties

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TypeScript provides a shorthand syntax called parameter properties.

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Access modifiers can be applied directly to constructor parameters, automatically creating and initializing properties.

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Example

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// Define the Point class\\n\\nclass Point {\\n\\n  // Apply modifiers directly to constructor parameters\\n\\n  // Equivalent to declaring and initializing properties simultaneously\\n\\n  constructor(\\n\\n    // public modifier: creates public property x\\n\\n    public x: number,\\n\\n    // public modifier: creates public property y\\n\\n    public y: number,\\n\\n    // private modifier: creates private property z\\n\\n    private z: number\\n\\n  ){\\n\\n    // Constructor body can be empty; properties are auto-created\\n\\n  }\\n\\n  // Compute 3D sum\\n\\n  public sum(): number {\\n\\n    // All properties accessible within the class\\n\\n    return this.x+this.y+this.z;\\n\\n  }\\n\\n}\\n\\n// Create a point instance\\n\\nvar point =new Point(1,2,3);\\n\\n// Public properties accessible from outside\\n\\nconsole.log("x: "+ point.x);\\n\\nconsole.log("y: "+ point.y);\\n\\n// Call method\\n\\nconsole.log("Sum: "+ point.sum());\\n\\n// Error: Private property inaccessible from outside\\n\\n// console.log(point.z); // Compilation error!\\n

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Output:

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x: 1 y: 2Sum: 6\\n

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Code Simplification: Parameter properties significantly simplify class definitions, avoiding repetitive declaration and assignment code.
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Comparison of Access Modifiers

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The table below compares the accessibility scopes of the three access modifiers.

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Modifier	Within Class	Subclasses	Outside Class (Instance)
`public`	✓ Accessible	✓ Accessible	✓ Accessible
`protected`	✓ Accessible	✓ Accessible	✗ Not Accessible
`private`	✓ Accessible	✗ Not Accessible	✗ Not Accessible

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Recommendation: By default, use private; use protected when subclasses need access; use public only when full exposure is required.
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Notes

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Default Modifier: If no modifier is specified, public is used by default.
Constructor: Constructors can also use access modifiers to control instantiation permissions.
readonly Combination: readonly can be combined with public, protected, and private.
Compilation Impact: Access modifiers are enforced only at compile time; they have no effect at runtime.

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Best Practice: Use the most restrictive access modifier possible. Expose only necessary public interfaces, and keep internal implementation private or protected.
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Summary

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Access modifiers are the core tools TypeScript uses to implement encapsulation.

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public: Publicly accessible; the default value.
private: Private; visible only within the defining class.
protected: Protected; visible within the defining class and its subclasses.
readonly: Read-only; cannot be modified after initialization.
Parameter Properties: A shorthand syntax for simplified property declaration.

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Recommendation: Develop the habit of using access modifiers—it is foundational to writing high-quality TypeScript code.
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YouTip

Ts Access Modifiers

TypeScript Access Modifiers |

Why Access Modifiers Are Needed

public Modifier

Example

private Modifier

Example

protected Modifier

Example

readonly Modifier

Example

Parameter Properties

Example

Comparison of Access Modifiers

Notes

Summary

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