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What's the difference between map() and forEach()? How does filter() work? What's the difference between splice() and slice()? How do find() and findIndex() differ from filter()? What does Array.isArray() solve that typeof can't? How does sort() compare values by default, and how do you fix numeric sorting? What is the difference between flat() and flatMap()? Why can array holes (sparse arrays) behave unexpectedly with iteration methods? How do you efficiently remove duplicate values from an array?
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What does reduce() do and what are its arguments?

Beginner

Answer

reduce() runs a callback that accumulates a single value across the array; its callback arguments are (accumulator, currentValue, index, array), plus an optional initial value for the accumulator.
const total = [1,2,3].reduce((sum, n) => sum + n, 0); // 6
Real-world example Summing a shopping cart's item prices into a single total.

Common follow-ups: What happens if you omit the initial value on an empty array?

Functional Programming
All 10 Arrays & Array Methods questions