Calculator.js

function Calculator(){
	
	this.add = function(num1, num2){
		if(this.isNumber([num1,num2]) == false){
			throw new Error('Invalid argument specified to add method.');
		}else{
			return num1 + num2;
		}
	}

	this.subtract = function(num1, num2){
		if(this.isNumber([num1,num2]) == false){
			throw new Error('Invalid argument specified to subtract method.');
		}else{
			return num1 - num2;
		}
	}

	this.isNumber = function(arr){
		for(var i=0; i < arr.length; i++){
			if(typeof(arr[i]) !== 'number'){
				return false;
			}
		}
		return true;
	}

}

var calc = new Calculator();
console.log(calc.add(1,"2"));
console.log(calc.subtract(5,4));

CalculatorSpec.js

describe('Calculator', function() {

    var calc = new Calculator();

    describe('The add method', function(){
    
        it('should add two numbers together', function(){
            expect(calc.add(1,1)).toEqual(2);
            expect(calc.add(20,33)).toEqual(53);
        });

        it('should throw an expection when a non number is passed', function(){
            expect( function(){ calc.add(1,"1") } ).toThrow(new Error("Invalid argument specified to add method."));
        });

    });

    describe('The subtract method', function(){
    
        it('should subtract two numbers together', function(){
            expect(calc.subtract(5,4)).toEqual(1);
            expect(calc.subtract(33,20)).toEqual(13);
        });

        it('should throw an expection when a non number is passed', function(){
            expect( function(){ calc.subtract(5,"2") } ).toThrow(new Error("Invalid argument specified to subtract method."));
        });

    });

    describe('The validator method', function(){
    
        it('return true when all numbers are passed in', function(){
            expect(calc.isNumber([5,7,3,5,6])).toBeTruthy();
            expect(calc.isNumber([5,66,7777,88,5])).toBeTruthy();
        });

        it('return false when a non number is passed in', function(){
            expect(calc.isNumber(["5","7","3","5","6"])).toBeFalsy();
            expect(calc.isNumber([5,66,"7777",88,5])).toBeFalsy();
        });

    });

});