Can you explain the distinctions between the private keyword and private fields in TypeScript?

Question

Can you explain the distinctions between the private keyword and private fields in TypeScript?

When using TypeScript 3.8+, what differences should be considered between utilizing the private keyword to designate a member private, as shown below:

class PrivateKeywordClass {
    private value = 1;
}

And opting for the # private fields proposed for JavaScript in the code snippet below:

class PrivateFieldClass {
    #value = 1;
}

Is there a preference for one method over the other?

typescript class private encapsulation class-fields

Answer 1

Answer №1

Understanding the Private Keyword

Exploring the concept of the private keyword in TypeScript reveals its role as a compile-time annotation. Specifically, it indicates that a property is restricted to access within a specific class:

class PrivateKeywordClass {
    private value = 1;
}

const obj = new PrivateKeywordClass();
obj.value // Results in compiler error: Property 'value' is private and only accessible within class 'PrivateKeywordClass'.

Despite the compile-time enforcement, bypassing this restriction is feasible by manipulating the type information:

const obj = new PrivateKeywordClass();
(obj as any).value // No compile error detected

It's important to note that the private keyword lacks runtime enforcement.

Examining the Emitted JavaScript

During the TypeScript-to-JavaScript compilation process, the private keyword is essentially discarded:

class PrivateKeywordClass {
    private value = 1;
}

Transformed into:

class PrivateKeywordClass {
    constructor() {
        this.value = 1;
    }
}

This transformation demonstrates why the private keyword fails to provide runtime safeguarding – in the resulting JavaScript code, it simply becomes a standard property.

Introducing Private Fields

Unlike the private fields, which ensure property privacy at runtime:

class PrivateFieldClass {
    #value = 1;

    getValue() { return this.#value; }
}

const obj = new PrivateFieldClass();

// Accessing '#value' outside the class context leads to unavailability
obj.value === undefined // Incorrect field reference
obj.getValue() === 1 // However, the class itself can access the private field!

// Attempting to access a private field from outside a class triggers a runtime syntax error:
obj.#value

// Accessing private fields of another class leads to a runtime type error
class Other {
    #value;

    getValue(obj) {
        return obj.#value // Error: Read of private field #value from an object which did not contain the field
    }
}

new Other().getValue(new PrivateKeywordClass());

When utilizing private fields in TypeScript and targeting ES2021 or older JavaScript versions, TypeScript synthesizes code to mimic private field behavior at runtime using a WeakMap.

class PrivateFieldClass {
    constructor() {
        _x.set(this, 1);
    }
}
_x = new WeakMap();

For ES2021 and newer targets, TypeScript emits the private field directly:

class PrivateFieldClass {
    constructor() {
        this.#x = 1;
    }
    #x;
}

Choosing Between Private Keyword and Private Fields

The decision between the private keyword and private fields hinges on the specific requirements of your project.

The private keyword serves as a reliable default option, fulfilling its intended purpose effectively and exhibiting a track record of successful usage within the TypeScript community. Switching an entire codebase to adopt private fields may not be necessary, especially if the target is not esnext, as private fields in TypeScript emit JavaScript code that could impact performance. Additionally, nuances distinguish private fields from the private keyword.

However, if runtime privacy enforcement or outputting esnext JavaScript is essential, opting for private fields is recommended.

Moreover, evolving community norms and conventions regarding the usage of private fields versus the private keyword highlight the dynamic nature of these features within the JavaScript/TypeScript ecosystems.

Key Distinctions to Note

Private fields evade detection by methods like Object.getOwnPropertyNames and are not serialized by JSON.stringify
Inheritance intricacies exist, with TypeScript prohibiting the declaration of a private property in a subclass sharing the name with a private property in the superclass for the private keyword but not for private fields
A private keyword private property with no initializer does not result in a property declaration in the compiled JavaScript, unlike private fields

Further Resources:

Answer 2

Understanding the Private Keyword

Exploring the concept of the private keyword in TypeScript reveals its role as a compile-time annotation. Specifically, it indicates that a property is restricted to access within a specific class:

class PrivateKeywordClass {
    private value = 1;
}

const obj = new PrivateKeywordClass();
obj.value // Results in compiler error: Property 'value' is private and only accessible within class 'PrivateKeywordClass'.

Despite the compile-time enforcement, bypassing this restriction is feasible by manipulating the type information:

const obj = new PrivateKeywordClass();
(obj as any).value // No compile error detected

It's important to note that the private keyword lacks runtime enforcement.

Examining the Emitted JavaScript

During the TypeScript-to-JavaScript compilation process, the private keyword is essentially discarded:

class PrivateKeywordClass {
    private value = 1;
}

Transformed into:

class PrivateKeywordClass {
    constructor() {
        this.value = 1;
    }
}

This transformation demonstrates why the private keyword fails to provide runtime safeguarding – in the resulting JavaScript code, it simply becomes a standard property.

Introducing Private Fields

Unlike the private fields, which ensure property privacy at runtime:

class PrivateFieldClass {
    #value = 1;

    getValue() { return this.#value; }
}

const obj = new PrivateFieldClass();

// Accessing '#value' outside the class context leads to unavailability
obj.value === undefined // Incorrect field reference
obj.getValue() === 1 // However, the class itself can access the private field!

// Attempting to access a private field from outside a class triggers a runtime syntax error:
obj.#value

// Accessing private fields of another class leads to a runtime type error
class Other {
    #value;

    getValue(obj) {
        return obj.#value // Error: Read of private field #value from an object which did not contain the field
    }
}

new Other().getValue(new PrivateKeywordClass());

When utilizing private fields in TypeScript and targeting ES2021 or older JavaScript versions, TypeScript synthesizes code to mimic private field behavior at runtime using a WeakMap.

class PrivateFieldClass {
    constructor() {
        _x.set(this, 1);
    }
}
_x = new WeakMap();

For ES2021 and newer targets, TypeScript emits the private field directly:

class PrivateFieldClass {
    constructor() {
        this.#x = 1;
    }
    #x;
}

Choosing Between Private Keyword and Private Fields

The decision between the private keyword and private fields hinges on the specific requirements of your project.

The private keyword serves as a reliable default option, fulfilling its intended purpose effectively and exhibiting a track record of successful usage within the TypeScript community. Switching an entire codebase to adopt private fields may not be necessary, especially if the target is not esnext, as private fields in TypeScript emit JavaScript code that could impact performance. Additionally, nuances distinguish private fields from the private keyword.

However, if runtime privacy enforcement or outputting esnext JavaScript is essential, opting for private fields is recommended.

Moreover, evolving community norms and conventions regarding the usage of private fields versus the private keyword highlight the dynamic nature of these features within the JavaScript/TypeScript ecosystems.

Key Distinctions to Note

Private fields evade detection by methods like Object.getOwnPropertyNames and are not serialized by JSON.stringify
Inheritance intricacies exist, with TypeScript prohibiting the declaration of a private property in a subclass sharing the name with a private property in the superclass for the private keyword but not for private fields
A private keyword private property with no initializer does not result in a property declaration in the compiled JavaScript, unlike private fields

Further Resources:

Answer 3

Answer №2

Use cases: Implementing Privacy with Private Fields

Introduction:

TC39 proposal for class fields
Synonyms: Private fields, hard private, run-time private

Ensuring Privacy at Compile-time and Run-time

Private fields in classes offer a way to maintain privacy both at compile-time and run-time, making it difficult for external entities to access class members. This prevents unauthorized access to class properties in a way that is not easily bypassed.

class A {
    #a: number;
    constructor(a: number) {
        this.#a = a;
    }
}

let foo: A = new A(42);
foo.#a; // error, cannot access outside class body
(foo as any).#bar; // still restricted

Secure Class Inheritance

The use of private fields ensures a unique scope, preventing accidental overwrites of private properties with similar names in class hierarchies.

class A { 
    #a = "a";
    fnA() { return this.#a; }
}

class B extends A {
    #a = "b"; 
    fnB() { return this.#a; }
}

const b = new B();
b.fnA(); // returns "a" - private property in A is preserved
b.fnB(); // returns "b"

The TypeScript compiler issues an error when private properties are at risk of being overwritten. However, ignoring compile errors and utilizing emitted JavaScript code can still enable access at run-time.

Use in External Libraries

Library authors can refactor private identifiers without breaking changes for clients, while users are protected from internal field access.

Hidden from JavaScript API

Built-in JavaScript functions and methods do not recognize private fields, providing a more predictable property selection at run-time.

Use cases: TypeScript's Private Keyword

Introduction:

Private keyword in TypeScript docs
Synonyms: TypeScript private, soft private, compile-time private

Compile-time Only Privacy for Internal Class Access

Private members in TypeScript act as conventional properties at run-time, allowing flexible access to class internals from external contexts with type assertions or dynamic property access.

class A { 
    constructor(private a: number) { }
}

const a = new A(10);
a.a; // TypeScript error
(a as any).a; // works
const casted: any = a; casted.a // works

TS even allows dynamic property access of a private member with an escape-hatch:

class C {
  private foo = 10;
}

const res = new C()["foo"]; // 10, res has type number

Accessing private variables may be useful for unit tests, debugging, or advanced scenarios involving project-internal classes.

Cross-Instance Access

Instances of the same class can access private members of other instances, allowing for shared information within the class.

Sources

Answer 4

Use cases: Implementing Privacy with Private Fields

Introduction:

TC39 proposal for class fields
Synonyms: Private fields, hard private, run-time private

Ensuring Privacy at Compile-time and Run-time

Private fields in classes offer a way to maintain privacy both at compile-time and run-time, making it difficult for external entities to access class members. This prevents unauthorized access to class properties in a way that is not easily bypassed.

class A {
    #a: number;
    constructor(a: number) {
        this.#a = a;
    }
}

let foo: A = new A(42);
foo.#a; // error, cannot access outside class body
(foo as any).#bar; // still restricted

Secure Class Inheritance

The use of private fields ensures a unique scope, preventing accidental overwrites of private properties with similar names in class hierarchies.

class A { 
    #a = "a";
    fnA() { return this.#a; }
}

class B extends A {
    #a = "b"; 
    fnB() { return this.#a; }
}

const b = new B();
b.fnA(); // returns "a" - private property in A is preserved
b.fnB(); // returns "b"

The TypeScript compiler issues an error when private properties are at risk of being overwritten. However, ignoring compile errors and utilizing emitted JavaScript code can still enable access at run-time.

Use in External Libraries

Library authors can refactor private identifiers without breaking changes for clients, while users are protected from internal field access.

Hidden from JavaScript API

Built-in JavaScript functions and methods do not recognize private fields, providing a more predictable property selection at run-time.

Use cases: TypeScript's Private Keyword

Introduction:

Private keyword in TypeScript docs
Synonyms: TypeScript private, soft private, compile-time private

Compile-time Only Privacy for Internal Class Access

Private members in TypeScript act as conventional properties at run-time, allowing flexible access to class internals from external contexts with type assertions or dynamic property access.

class A { 
    constructor(private a: number) { }
}

const a = new A(10);
a.a; // TypeScript error
(a as any).a; // works
const casted: any = a; casted.a // works

TS even allows dynamic property access of a private member with an escape-hatch:

class C {
  private foo = 10;
}

const res = new C()["foo"]; // 10, res has type number

Accessing private variables may be useful for unit tests, debugging, or advanced scenarios involving project-internal classes.

Cross-Instance Access

Instances of the same class can access private members of other instances, allowing for shared information within the class.

Sources

Can you explain the distinctions between the private keyword and private fields in TypeScript?

Answer №1

Understanding the Private Keyword

Examining the Emitted JavaScript

Introducing Private Fields

Choosing Between Private Keyword and Private Fields

Key Distinctions to Note

Further Resources:

Answer №2

Use cases: Implementing Privacy with Private Fields

Ensuring Privacy at Compile-time and Run-time

Secure Class Inheritance

Use in External Libraries

Hidden from JavaScript API

Use cases: TypeScript's Private Keyword

Compile-time Only Privacy for Internal Class Access

Cross-Instance Access

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