JavaScript Array Methods

Modern JavaScript has powerful array methods that take a function (a callback) and run it for each item. They replace many loops.

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map: change every item

const prices = [100, 200, 300];
const withTax = prices.map(p => p * 1.18);
console.log(withTax);

map() returns a new array of the same length.

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filter: keep some items

const ages = [12, 18, 25, 9, 40];
const adults = ages.filter(a => a >= 18);
console.log(adults);

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reduce: combine into one value

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

The second argument (0) is the starting value of total.

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find, some and every

const users = [
  { id: 1, name: "Robin" },
  { id: 2, name: "Maria" }
];

console.log(users.find(u => u.id === 2));
console.log([1, 2, 3].some(n => n > 2));
console.log([1, 2, 3].every(n => n > 2));

Method	Returns
find()	The first item that matches, or undefined
findIndex()	Its index, or -1
some()	true if at least one item matches
every()	true if all items match

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sort and reverse

sort() changes the original array, and sorts as text by default. For numbers give a compare function.

const nums = [10, 9, 1];
console.log([...nums].sort());
console.log([...nums].sort((a, b) => a - b));
console.log([...nums].reverse());

The text sort puts 10 before 9, because “1” comes before “9”. We copy the array with [...nums], so the original is not changed.

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Chain methods

const result = [1, 2, 3, 4, 5, 6]
  .filter(n => n % 2 === 0)
  .map(n => n * 10);
console.log(result);

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Summary

• map transforms, filter selects, reduce combines.
• find, some and every search.
• sort needs a compare function for numbers.
• Next: JavaScript Objects.

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