1. Condensing an Array

The .reduce() method executes a user-supplied "reducer" callback function on each element of the array, passing in the return value from the calculation on the preceding element.

2. Why It Matters

.reduce() is the swiss army knife of array methods. It can compute total sums, flatten nested arrays, or build complex objects from array streams in a single pass.

3. The Reducer Pattern

Pass a callback function taking (accumulator, currentValue) and an explicit initial accumulator starting value:

const cartPrices = [15, 25, 10];

// Sum total prices with initial accumulator 0
const totalCost = cartPrices.reduce((sum, price) => {
  return sum + price;
}, 0);

console.log(totalCost); // 50

4. Building Objects with .reduce()

You can use .reduce() with an empty object {} initial value to count occurrences or group items:

const votes = ["yes", "no", "yes", "yes", "no"];

const tally = votes.reduce((acc, vote) => {
  acc[vote] = (acc[vote] || 0) + 1;
  return acc; // Must return the updated accumulator!
}, {});

console.log(tally); // { yes: 3, no: 2 }

5. Common Mistakes

• Omitting the Initial Accumulator Value: If you omit the initial value argument in .reduce(), array element 0 becomes the initial accumulator and iteration starts at index 1. On empty arrays, omitting the initial value throws an uncaught TypeError: Reduce of empty array with no initial value!
• Forgetting to Return Accumulator: The reducer function MUST return the updated accumulator at the end of every step.

6. Try It Yourself

Write it yourself. The tests below run against your code.

function sumArray(numbers) {
  return 0;
}

7. Next Steps

Always supply an initial accumulator value to .reduce(). Next, let's explore clean variable unpacking with Destructuring!