Lesson 16 — Array reduce

Fold many values into one

reduce(fn, start) walks the array and carries an accumulator — the result so far. For each item it calls fn(acc, item) and the return value becomes the new acc. The second argument is the starting value; for sums use 0.

[10, 25].reduce((acc, p) => acc + p, 0);   // 0 → 10 → 35

You can also find the biggest value: return whichever of acc and p is larger.

Why it matters: totals, maximums and averages are all one reduce away.