Interview question

How do abstract-like classes and interfaces work in JavaScript, since it doesn't natively support them? JavaScript में abstract-jaisi classes और interfaces कैसे काम करते हैं, जबकि native support नहीं है?

Answer

JavaScript has no built-in 'abstract class' or 'interface' keywords, but the same design intentions can be enforced manually using constructor checks and method existence checks.

// Simulating an abstract class - prevents direct instantiation
class Shape {
    constructor() {
        if (new.target === Shape) {
            // new.target refers to the constructor actually called with 'new'
            throw new Error('Shape is abstract and cannot be instantiated directly');
        }
    }

    area() {
        throw new Error('area() must be implemented by subclass'); // enforced 'abstract method'
    }
}

class Circle extends Shape {
    constructor(radius) {
        super();
        this.radius = radius;
    }
    area() { // required override
        return Math.PI * this.radius ** 2;
    }
}

// new Shape(); // Error: Shape is abstract and cannot be instantiated directly
const circle = new Circle(5);
console.log(circle.area()); // 78.53...

class Triangle extends Shape {
    // Forgot to implement area()
}
const triangle = new Triangle();
// triangle.area(); // throws 'area() must be implemented by subclass'

// Simulating an interface via a checking function (duck typing)
function implementsInterface(obj, methods) {
    return methods.every(method => typeof obj[method] === 'function');
}

class FileLogger {
    log(msg) { console.log('File:', msg); }
    error(msg) { console.log('File error:', msg); }
}

const logger = new FileLogger();
console.log(implementsInterface(logger, ['log', 'error'])); // true

// TypeScript's interfaces solve this problem at COMPILE time instead;
// in plain JS, these runtime checks are the closest equivalent

JavaScript में built-in 'abstract class' या 'interface' keywords नहीं हैं, पर वही design intentions constructor checks और method existence checks से manually enforce की जा सकती हैं।

// Abstract class simulate करना
class Shape {
    constructor() {
        if (new.target === Shape) {
            throw new Error('Shape abstract है, directly instantiate नहीं हो सकता');
        }
    }

    area() {
        throw new Error('area() subclass में implement होना चाहिए');
    }
}

class Circle extends Shape {
    constructor(radius) {
        super();
        this.radius = radius;
    }
    area() {
        return Math.PI * this.radius ** 2;
    }
}

// new Shape(); // Error
const circle = new Circle(5);
console.log(circle.area()); // 78.53...

class Triangle extends Shape {
    // area() implement करना भूल गए
}
const triangle = new Triangle();
// triangle.area(); // Error throw होगा

// Interface simulate करना - duck typing
function implementsInterface(obj, methods) {
    return methods.every(method => typeof obj[method] === 'function');
}

class FileLogger {
    log(msg) { console.log('File:', msg); }
    error(msg) { console.log('File error:', msg); }
}

const logger = new FileLogger();
console.log(implementsInterface(logger, ['log', 'error'])); // true

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