Where this matters
A running total is useful for adding quantities in a cart. The initial accumulator determines where the calculation starts.
JavaScript · read-only example
console.log([2, 3, 5].reduce((sum, value) => sum + value, 0));Read the full explanation
The accumulator starts at zero, becomes two, then five, then ten. Each returned sum is passed to the next callback.
Trace the code
- The explicit initial accumulator is 0.
- The callback returns 2, then 5, then 10 as each input value is added.
- Reduce returns the final accumulator, not a list of intermediate totals.
Why the other predictions differ
5— Five is an intermediate accumulator, not the final one.235— The operands are numbers, so + adds instead of concatenating strings.
10
Reading this explanation does not record mastery. A correct submitted attempt can do so, including after review.
Change one thing
Start the accumulator at 10, keeping the same array.
Predict the result, then run this self-contained snippet in your browser’s developer console. It uses only sample values and makes no network requests.
console.log([2, 3, 5].reduce((sum, value) => sum + value, 10));Check the variation’s output
20
Explain which binding, value, or operation changed before moving to the next challenge.