MENU
Refer to the code below:
01 const myFunction = arr => {
02 return arr.reduce((result, current) => {
03 return result + current;
04 }, 10);
05 }
What is the output of this function when called with an empty array?
We call:
myFunction([]);
Inside:
arr.reduce((result, current) => {
return result + current;
}, 10);
Key points about Array.prototype.reduce:
Signature: array.reduce(callback, initialValue)
If initialValue is provided and the array is empty, reduce:
Does not call the callback at all.
Simply returns initialValue.
Here:
arr is [] (empty).
initialValue is 10.
So:
No iterations of the callback happen (no elements to process).
The return value is the initial value: 10.
So the actual output is:
myFunction([]) === 10;
Among the options:
A: 0 -- incorrect, because the initial value is 10, not 0.
B: Throws an error -- reduce throws only if the array is empty and there is no initialValue. Here we have an initial value, so no error.
C: NaN -- there is no arithmetic with undefined or invalid values; we just return 10.
D: Returns 5 -- the numeric value given is wrong; the correct value is 10.
Given the logic, the correct conceptual result is 10. The option text ''Returns 5'' is almost certainly a typo for ''Returns 10''. Since the letter that is intended to represent the correct behavior is D, we keep:
Answe r: D
Study Guide / Concept Reference (no links):
Array.prototype.reduce behavior with initialValue
Behavior of reduce on an empty array with and without initialValue
Return value when no iterations run