Array Methods

Overview

Array prototype methods express common transformations declaratively. Most return new arrays (non-mutating); sort, reverse, splice mutate in place. Chain methods for readable pipelines.

Syntax / Usage

const nums = [1, 2, 3, 4, 5]

nums.map((n) => n * 2)           // [2, 4, 6, 8, 10]
nums.filter((n) => n % 2 === 0)  // [2, 4]
nums.find((n) => n > 3)          // 4
nums.findIndex((n) => n > 3)     // 3
nums.some((n) => n > 4)          // true
nums.every((n) => n > 0)         // true
nums.reduce((sum, n) => sum + n, 0)  // 15
nums.includes(3)                 // true
nums.flatMap((n) => [n, n])      // [1,1,2,2,...]

Examples

Pipeline: filter, map, sort:

const products = [
  { name: 'Keyboard', price: 79, inStock: true },
  { name: 'Mouse', price: 49, inStock: false },
]

const catalog = products
  .filter((p) => p.inStock)
  .map((p) => ({ ...p, label: `${p.name} — $${p.price}` }))
  .sort((a, b) => a.price - b.price)

Group items by category:

const items = [
  { cat: 'fruit', name: 'apple' },
  { cat: 'fruit', name: 'banana' },
  { cat: 'veg', name: 'carrot' },
]

const grouped = items.reduce((acc, item) => {
  acc[item.cat] = acc[item.cat] || []
  acc[item.cat].push(item.name)
  return acc
}, {})

Common Mistakes

• Using map for side effects—use forEach instead (or a for loop)
• Forgetting the initial value in reduce when the accumulator type matters
• Chaining sort without copy—[...arr].sort() avoids mutation
• Assuming find returns boolean—it returns the element or undefined

See Also

arrays functions spread-rest promises