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JavaScript65 min total · 16 parts

JavaScript Array and Object Methods Cheat Sheet

Contents — Part 15 of 16: A Worked Example Combining Several of These
  1. 1.Overview
  2. 2.Mutating vs. Non-Mutating: The Distinction That Matters Most
  3. 3.Transforming Arrays
  4. 4.Searching and Testing Arrays
  5. 5.Iterating Arrays
  6. 6.reduce() in Depth
  7. 7.Object Methods
  8. 8.Spread and Rest
  9. 9.Destructuring
  10. 10.Optional Chaining and Nullish Coalescing
  11. 11.Equality: == vs. ===, and Comparing Objects
  12. 12.Map and Set: Alternatives to Plain Objects
  13. 13.JSON.stringify and JSON.parse
  14. 14.Shallow Copy vs. Deep Copy
  15. 15.A Worked Example Combining Several of These
  16. 16.Common Mistakes Worth Remembering
Part 15 of 16 · ~1 min

A Worked Example Combining Several of These

Diagram of a filter then map pipeline turning a users array into a list of active adults' names

const users = [
  { name: "Alice", age: 30, active: true },
  { name: "Bob", age: 17, active: false },
  { name: "Carol", age: 25, active: true },
];

const activeAdultNames = users
  .filter(u => u.active && u.age >= 18)
  .map(u => u.name);
// ["Alice", "Carol"]

const byName = Object.fromEntries(users.map(u => [u.name, u]));
// { Alice: {...}, Bob: {...}, Carol: {...} } — quick lookup table by name

const averageAge = users.reduce((sum, u) => sum + u.age, 0) / users.length;
// 24 — reduce for a running total, then a plain division

This chain — filter, then map — is one of the most common patterns in real JavaScript code, and it reads almost exactly like the English description of what it does. Combining .reduce() with Object.fromEntries() for lookup tables, and chaining .filter().map() for transforming lists of records, covers a large share of the data-shaping code in a typical frontend or Node.js codebase.

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Common Mistakes Worth Remembering