This code will output `6`. The function `sum` uses the rest parameter syntax (`...numbers`) to collect all arguments passed to the function into an array called `numbers`. In this case, when calling `sum(1, 2, 3)`, the `numbers` array becomes `[1, 2, 3]`. The function then uses the `reduce` method to add up all numbers in the array, starting with an initial total of 0. The calculation is 0 + 1 = 1, then 1 + 2 = 3, then 3 + 3 = 6. The rest parameter syntax is perfect for variadic functions (functions that can accept any number of arguments). Unlike the older `arguments` object, the rest parameter gives you a real array with all array methods like `map`, `filter`, and `reduce` available. The rest parameter must be the last parameter in a function definition, as it collects all remaining arguments.