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JavaScript Tutorial

Variables to closures, promises, the DOM and OOP — a complete JS reference.

Array.prototype.reduce()

Folds an array down to a single value by running an accumulator function over each element.

const nums = [1, 2, 3, 4];
const sum = nums.reduce((acc, n) => acc + n, 0);
// sum: 10

const grouped = ['a', 'b', 'a'].reduce((acc, val) => {
  acc[val] = (acc[val] || 0) + 1;
  return acc;
}, {});
// grouped: { a: 2, b: 1 }

Signature: arr.reduce((accumulator, current, index, array) => newAcc, initialValue)
ALWAYS pass an initial value to avoid subtle bugs on empty arrays
Powerful enough to implement map/filter/sum/groupBy yourself

Tip

Be ready to whiteboard 'implement map using reduce' — a very common follow-up.