Array.prototype.reduce() — JS Technical Interview Questions & Answers

Folds an array down to a single accumulated value.

Array.prototype.reduce()

  • Array.prototype.reduce() is a JS topic. In plain words you use it for fail students only in a marks list. Don’t start with a slogan — start with that picture.
  • Smallest example: filter(m => m < 40). Type it, run it, and say what you see. If you can do that from memory, you know Array.prototype.reduce().
  • From the example next to this theory: Folds an array down to a single accumulated value.
  • Trap: mutating while filtering. Fix that before you talk about advanced JS.
  • Viva: what is Array.prototype.reduce()? Then show filter(m => m < 40). Then name the trap.

Technical Interview Tip: What is Array.prototype.reduce()? Show this: filter(m => m < 40). Trap: mutating while filtering.

Code Examples for Array.prototype.reduce()

Array.prototype.reduce() — working example

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

Array.prototype.reduce() — reduce folds a list into one value; always pass an initial accumulator for clarity.

How to Explain Array.prototype.reduce() to an Interviewer

What is Array.prototype.reduce()?

Used for fail students only a marks list Not just the heading

Give one small example of Array.prototype.reduce().

filter(m => m < 40) Folds an array down to a single accumulated value

What mistake do beginners make with Array.prototype.reduce()?

mutating while filtering Show the fix

Where do you use Array.prototype.reduce()?

a marks list fail students only