Here is a simple approach using JavaScript `for` loops. We will loop starting from `x` up to `y` and keep reducing `y` by 1 every time the loop completes. We will store all the sums in variable `total`.
```javascript
function rangeSum(x, y) {
var total = 0;
while(y >= x) {
for(let i = x; i <= y; i++) {
total += i;
}
y--;
}
return total;
}
console.log(rangeSum(0, 3)); // Outputs: 10
```
In this function `rangeSum`, the outer `while` loop will keep decreasing `y` till `y` is not less than `x`. The inner `for` loop will keep adding `i` to `total` from `x` to `y`. This way you can get the sum of the range of numbers between `x` and `y` inclusive, and repeating the calculation after decrementing `y`.
This function should give you the desired output. However, keep in mind that this function has a time complexity of `O(n^2)` due to the nested loops. Practicing with such simple functions will give you a good grasp over loops and conditionals in JavaScript. Happy Coding!