Develop a prototype function in ES6/ESNext with a distinct scope (avoid using an inline function)

Question

Develop a prototype function in ES6/ESNext with a distinct scope (avoid using an inline function)

Consider the following example:

class Car {
    constructor(name) {
        this.kind = 'Car';
        this.name = name;
    }

    printName() {
        console.log('this.name');
    }
}

My goal is to define printName using a different approach, like this:

class Car {
    constructor(name) {
        this.kind = 'Car';
        this.name = name;
    }

    // we aim to redefine printName with a different scope
    // the syntax is close, but not exactly correct
    printName: makePrintName(foo, bar, baz) 
}

where makePrintName is a functor, similar to this:

exports.makePrintName = function(foo, bar, baz){
   return function(){ ... }
};

Is this achievable with ES6? My editor and TypeScript are not approving of this

NOTE: Achieving this with ES5 was simple and looked like this:

var Car = function(){...};

Car.prototype.printName = makePrintName(foo, bar, baz);

Currently, the best solution using class syntax for me is:

const printName = makePrintName(foo, bar, baz);

class Car {
  constructor(){...}
  printName(){
    return printName.apply(this, arguments);
  }
}

However, this solution is not ideal. The limitations of using class syntax become evident when trying to achieve what was easily done with ES5 syntax. The ES6 class wrapper is, therefore, a leaky abstraction.

For a real-life use case, please refer to:

https://github.com/sumanjs/suman/blob/master/lib/test-suite-helpers/make-test-suite.ts#L171

The issue with using

TestBlock.prototype.startSuite = ...

is that in that scenario, I cannot simply return the class on line:

https://github.com/sumanjs/suman/blob/master/lib/test-suite-helpers/make-test-suite.ts#L67

javascript typescript ecmascript-6

Answer 1

Answer №1

In JavaScript and TypeScript, the ideal methods for accomplishing this task may vary due to differences in the typing systems. If printName is intended to be a prototype method rather than an instance method (which is advantageous for several reasons), there are limited options available.

One approach is to retrieve the prototype method via accessor and preferably cache it to a variable.

const cachedPrintName = makePrintName(foo, bar, baz);

class Car {
    ...
    get printName(): () => void {
        return cachedPrintName;
    }
}

Alternatively, it can be lazily evaluated:

let cachedPrintName;

class Car {
    ...
    get printName(): () => void {
        return cachedPrintName || cachedPrintName = makePrintName(foo, bar, baz);
    }
}

Another option is to assign it directly to the class prototype. In this case, it should be typed as a class property to ensure TypeScript recognizes the assignment:

class Car {
    ...
    printName(): () => void;
}

Car.prototype.printName = makePrintName(foo, bar, baz);

Here, () => void represents the type of the function returned by makePrintName.

In TypeScript, a more natural approach is to extend the prototype chain and introduce new or modified methods using mixin classes. This adds complexity to the JavaScript code but satisfies TypeScript's type requirements:

function makePrintNameMixin(foo, bar, baz){
  return function (Class) {
    return class extends Class {
      printName() {...}
    }
  }
}

const Car = makePrintNameMixin(foo, bar, baz)(
  class Car {
    constructor() {...}
  }
);

Using TypeScript decorator is not straightforward in this scenario due to the lack of support for class mutation at the moment. To prevent type errors, the class should be supplemented with an interface:

interface Car {
  printName: () => void;
}

@makePrintNameMixin(foo, bar, baz)
class Car {
    constructor() {...}
}

Answer 2

In JavaScript and TypeScript, the ideal methods for accomplishing this task may vary due to differences in the typing systems. If printName is intended to be a prototype method rather than an instance method (which is advantageous for several reasons), there are limited options available.

One approach is to retrieve the prototype method via accessor and preferably cache it to a variable.

const cachedPrintName = makePrintName(foo, bar, baz);

class Car {
    ...
    get printName(): () => void {
        return cachedPrintName;
    }
}

Alternatively, it can be lazily evaluated:

let cachedPrintName;

class Car {
    ...
    get printName(): () => void {
        return cachedPrintName || cachedPrintName = makePrintName(foo, bar, baz);
    }
}

Another option is to assign it directly to the class prototype. In this case, it should be typed as a class property to ensure TypeScript recognizes the assignment:

class Car {
    ...
    printName(): () => void;
}

Car.prototype.printName = makePrintName(foo, bar, baz);

Here, () => void represents the type of the function returned by makePrintName.

In TypeScript, a more natural approach is to extend the prototype chain and introduce new or modified methods using mixin classes. This adds complexity to the JavaScript code but satisfies TypeScript's type requirements:

function makePrintNameMixin(foo, bar, baz){
  return function (Class) {
    return class extends Class {
      printName() {...}
    }
  }
}

const Car = makePrintNameMixin(foo, bar, baz)(
  class Car {
    constructor() {...}
  }
);

Using TypeScript decorator is not straightforward in this scenario due to the lack of support for class mutation at the moment. To prevent type errors, the class should be supplemented with an interface:

interface Car {
  printName: () => void;
}

@makePrintNameMixin(foo, bar, baz)
class Car {
    constructor() {...}
}

Answer 3

Answer №2

Switch : with =

printName = makePrintName()

: is used for type notation.

Update
As mentioned in the comments, the above code will not alter the prototype. Instead, you can achieve this outside the class definition by using ES5 syntax:

// workaround: extracting function return type
const dummyPrintName = !true && makePrintName();
type PrintNameType = typeof dummyPrintName;

class Car {
    // ...
    printName: PrintNameType;
}
Car.prototype.printName = makePrintName();

Playground

Answer 4

Switch : with =

printName = makePrintName()

: is used for type notation.

Update
As mentioned in the comments, the above code will not alter the prototype. Instead, you can achieve this outside the class definition by using ES5 syntax:

// workaround: extracting function return type
const dummyPrintName = !true && makePrintName();
type PrintNameType = typeof dummyPrintName;

class Car {
    // ...
    printName: PrintNameType;
}
Car.prototype.printName = makePrintName();

Playground

Answer 5

Answer №3

One innovative approach that hasn't been discussed yet involves utilizing a method decorator. This particular decorator takes a method implementation and places it on the prototype as needed:

function setMethod<T extends Function>(value: T) {
    return function (target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>): void {
      descriptor.value = value;
      delete descriptor.get;
      delete descriptor.set;
    };
}

Consider storing this decorator in a library for future use. Below is an example of how you can apply it:

class Car {
    constructor(name) {
        this.kind = 'Car';
        this.name = name;
    }

    @setMethod(makePrintName(foo, bar, baz))
    printName() {} // dummy implementation
}

One caveat is that you must include a placeholder implementation of the method with the correct signature in the class, as the decorator requires something to decorate. Nevertheless, this approach performs exactly as intended during runtime (it is not an instance method that incurs a new function definition per instance, nor an accessor that requires an additional function call each time it is used).

Does this offer a helpful solution?

Answer 6

One innovative approach that hasn't been discussed yet involves utilizing a method decorator. This particular decorator takes a method implementation and places it on the prototype as needed:

function setMethod<T extends Function>(value: T) {
    return function (target: any, propertyKey: string, descriptor: TypedPropertyDescriptor<T>): void {
      descriptor.value = value;
      delete descriptor.get;
      delete descriptor.set;
    };
}

Consider storing this decorator in a library for future use. Below is an example of how you can apply it:

class Car {
    constructor(name) {
        this.kind = 'Car';
        this.name = name;
    }

    @setMethod(makePrintName(foo, bar, baz))
    printName() {} // dummy implementation
}

One caveat is that you must include a placeholder implementation of the method with the correct signature in the class, as the decorator requires something to decorate. Nevertheless, this approach performs exactly as intended during runtime (it is not an instance method that incurs a new function definition per instance, nor an accessor that requires an additional function call each time it is used).

Does this offer a helpful solution?

Answer 7

Answer №4

Would you consider giving this a shot?

function House() {
    this.type = 'House';
    this.rooms = 3;
    this.color = 'green';
}

Answer 8

Would you consider giving this a shot?

function House() {
    this.type = 'House';
    this.rooms = 3;
    this.color = 'green';
}

Answer 9

Answer №5

Am I missing something here?

The class keyword in TypeScript is essentially just a more convenient way of creating objects compared to the delegate prototype in ES5. I recently experimented with it and found it quite interesting.

class Vehicle {
private type: string;
private brand: string;
printBrand: Function;
    constructor(brand) {
                this.type = 'Vehicle';
                this.brand = brand;
            }

   }
var generatePrintBrand = function (foo, bar, baz) {
    return function () { console.log(foo, bar, baz); };
};
Vehicle.prototype.printBrand = generatePrintBrand('hi', 'there', 'how are you');
var toyota = new Vehicle('Toyota');
toyota.printBrand();
console.log(toyota.hasOwnProperty('printBrand'));

Answer 10

Am I missing something here?

The class keyword in TypeScript is essentially just a more convenient way of creating objects compared to the delegate prototype in ES5. I recently experimented with it and found it quite interesting.

class Vehicle {
private type: string;
private brand: string;
printBrand: Function;
    constructor(brand) {
                this.type = 'Vehicle';
                this.brand = brand;
            }

   }
var generatePrintBrand = function (foo, bar, baz) {
    return function () { console.log(foo, bar, baz); };
};
Vehicle.prototype.printBrand = generatePrintBrand('hi', 'there', 'how are you');
var toyota = new Vehicle('Toyota');
toyota.printBrand();
console.log(toyota.hasOwnProperty('printBrand'));

Develop a prototype function in ES6/ESNext with a distinct scope (avoid using an inline function)

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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