a.js

function Calculator(){
	
	this.add = function(num1, num2){
		if(this.isNumber([num1,num2]) == false){
			throw "error: invalid input to function.";
		}else{
			return num1 + num2;
		}
	}

	this.subtract = function(num1, num2){
		if(this.isNumber([num1,num2]) == false){
			throw "error: invalid input to function.";
		}else{
			return num1 - num2;
		}
	}

	this.isNumber = function(arr){
		//console.log(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"));