Array Methods – map, filter, reduce, find, some, every

Functional array helpers make data transformation declarative.

const nums = [1,2,3,4,5];
const evens = nums.filter(n=>n%2===0); // [2,4]
const squares = nums.map(n=>n*n);      // [1,4,9,16,25]
const sum = nums.reduce((a,b)=>a+b,0); // 15

Other useful methods:

• find – first element matching a predicate.
• some – any element matches?
• every – all elements match?

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Example: Group objects by property

const books = [
  {genre:'fiction',title:'A'},
  {genre:'nonfiction',title:'B'},
  {genre:'fiction',title:'C'}
];
const byGenre = books.reduce((acc,b)=>{
  (acc[b.genre] = acc[b.genre] || []).push(b);
  return acc;
},{});
console.log(byGenre);