The value of y after this code runs is 10. When the function is called with `test(5, undefined)`, the first parameter `x` gets the value 5, and the second parameter `y` receives `undefined`. Since `undefined` is explicitly passed for the parameter `y`, which has a default value of 10, the default value is applied. The function then returns this default value. This example demonstrates how default parameters behave when `undefined` is explicitly passed—they work exactly the same as if no value was provided at all. This is useful in scenarios where you might need to use the default for one parameter but not others, or when working with function calls where some arguments might be dynamically generated as `undefined`.