Chainable calculator

Write calculator(start). It returns a calculator object that holds a running total, starting at start, which defaults to 0. Every calculator has its own total.

The calculator has four methods that each take a number: add(n) adds n to the total, subtract(n) subtracts it, multiply(n) multiplies the total by it, and divide(n) divides the total by it. Each one returns the calculator itself, so calls can be chained, and each works on the total left by the calls before it.

value() returns the current total.

divide(0) throws a RangeError and leaves the total as it was.

calculator(2).add(3).multiply(4).value();           // 20: (2 + 3) * 4
calculator().add(10).subtract(4).divide(2).value(); // 3
 
const calc = calculator(1);
calc.add(2) === calc; // true: every method returns the calculator
calc.multiply(3);
calc.value();         // 9
calc.divide(0);       // throws a RangeError
calc.value();         // 9

What the tests check

1. 1value() returns the starting total

   expect(calculator(5).value()).toBe(5);
   expect(calculator(-2.5).value()).toBe(-2.5);
   

2. 2starts at 0 by default

   expect(calculator().value()).toBe(0);
   expect(calculator().add(4).value()).toBe(4);
   

3. 3applies each step to the total so far, in order

   expect(calculator(2).add(3).multiply(4).value()).toBe(20);
   expect(calculator(2).multiply(4).add(3).value()).toBe(11);
   expect(calculator().add(10).subtract(4).divide(2).value()).toBe(3);
   

4. 4every method returns the calculator itself

   const calc = calculator(1);
   expect(calc.add(1)).toBe(calc);
   expect(calc.subtract(1)).toBe(calc);
   expect(calc.multiply(2)).toBe(calc);
   expect(calc.divide(2)).toBe(calc);
   

5. 5steps made in separate statements add up

   const calc = calculator(1);
   calc.add(2);
   calc.multiply(3);
   expect(calc.value()).toBe(9);
   calc.subtract(1).divide(4);
   expect(calc.value()).toBe(2);
   

6. 6calculators don't share a total

   const a = calculator(1);
   const b = calculator(100);
   a.add(1);
   b.subtract(50);
   expect(a.value()).toBe(2);
   expect(b.value()).toBe(50);
   

7. 7divide(0) throws a RangeError and keeps the total

   const calc = calculator(6);
   expect(() => calc.divide(0)).toThrow(RangeError);
   expect(calc.value()).toBe(6);
   expect(calc.divide(2).value()).toBe(3);
   

Learn more

• Methods & thisLesson
• ClosuresLesson
• Write calculator(start), whose methods can be chained: calculator(2).add(3).multiply(4).value().Interview question