Problem applying Decorator Design Pattern to JavaScript code

Question

I have a class named Calculation which I added two functions are addOperationToName() and sayOperation() as examples of the Decorator design pattern. However, when I test it using Jest, I got an error that 'fn is not a function'. Can someone help me?

Calculation.js

class Calculation {
constructor(a, b, op) {
    this.a = a;
    this.b = b;
    this.op = op;
}

static Create(a, b, op){
    return new Calculation(a, b, op);
}

 GetResults() {
    return this.op(this.a,this.b)
}

addOperationToName(fn){
    return function(name){
        const operation = name + ' is a operation';
        fn(operation);
    }
}

sayOperation(name){
    console.log(name);
}
}
module.exports = Calculation;

Calculation.test.js

const Calculation = require('../src/models/Calculation');

test('Test say name for Product operation', () => {
let op = Product;
let calculation = new Calculation(1,2,op);
let sayTest = calculation.addOperationToName(calculation.sayOperation());
expect(sayTest('Multiply')).toBe('Multiply is an operation');
});

Did you mean to reference calculation.sayOperation? In the scope of the Jest test, sayOperation has not been declared which is why you are getting that error. — Jimi
– Jimi, Commented Feb 27, 2021 at 22:23
I did change it to calculation.sayOperation and there is an error said undefined — BeefNoodle
– BeefNoodle, Commented Feb 27, 2021 at 22:45

dave · Accepted Answer · 2021-02-28 02:00:34Z

1

You are supposed to pass a function to addOperationToName, right now you are returning the result of a function - just change calculation.addOperationToName(calculation.sayOperation()); to calculation.addOperationToName(calculation.sayOperation);

class Calculation {
    constructor(a, b, op) {
        this.a = a;
        this.b = b;
        this.op = op;
    }

    static Create(a, b, op){
        return new Calculation(a, b, op);
    }

     GetResults() {
        return this.op(this.a,this.b)
    }

    addOperationToName(fn){
        return function(name){
            const operation = name + ' is a operation';
            return fn(operation);
        }
    }

    sayOperation(name){
        console.log(name);
        return name;
    }
}

let calculation = new Calculation(1,2,null);
let sayTest = calculation.addOperationToName(calculation.sayOperation);
console.log(sayTest('Multiply') === 'Multiply is a operation')

edited Feb 28, 2021 at 2:00

answered Feb 28, 2021 at 0:24

dave

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2 Comments

BeefNoodle Over a year ago

I did change it but there is an error said undefined

dave Over a year ago

see edit - you need to return the results of addOperationToName and of sayOperation, previously it was just console.log-ing it

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