simple_sum.js

var totalSum = 0; // this is variable with global scope

function sumTwoNumbers(x, y) {
    if(!y) { // this checks if y is undefined
        // if y is undefined, it will not
        // enter if block
        totalSum = totalSum + x;
    }else {
        totalSum = x + y;
        // here total sum becomes 300
        // and totalSum is global by default
    }
}
// 100 + 200 + 100 + 300 + 500 + 700 = 1900
sumTwoNumbers(100, 200);
sumTwoNumbers(100);
sumTwoNumbers(300);
sumTwoNumbers(500);
sumTwoNumbers(700);
console.log(totalSum);