array.md

[JavaScript] - Array Methods

SOURCE

Simple Methods

let arr = ['a', 'b', 'c', 'd', 'e'];

// SLICE
console.log(arr.slice(2)); // ['c', 'd', 'e'], create a new aray starting at index index 2 and doesn't mutate the original array
console.log(arr.slice(2, 4)); // ['c', 'd'], start at index 2 and end at 4, 4 is exclusive
console.log(arr.slice(-2)); // ['d', 'e'], take the last 2 elements
console.log(arr.slice(-1)); // ['e']
console.log(arr.slice(1, -2)); // ['b', 'c']
console.log(arr.slice()); // create a shallow copy of the original array
console.log([...arr]); // this also create a shallow copy. Which one to use is up to you

// SPLICE
// this mutates the original array
// console.log(arr.splice(2));
arr.splice(-1); // remove the last element
console.log(arr); // ['a', 'b', 'c', 'd']
arr.splice(1, 2); // remove 2 elements after position 1
console.log(arr); // ['a', 'd']

// REVERSE
arr = ['a', 'b', 'c', 'd', 'e'];
const arr2 = ['j', 'i', 'h', 'g', 'f'];
console.log(arr2.reverse()); // this also mutate the original array
console.log(arr2);

// CONCAT
const letters = arr.concat(arr2); // this doesn't mutate the original arrays
console.log(letters);
console.log([...arr, ...arr2]); // this also doesn't mutate the original arrays. Which one to use is up to you

// JOIN
console.log(letters.join(' - '));

The new at

const arr3 = [23, 11, 64];
console.log(arr3[0]);
console.log(arr3.at(0));

// getting last array element
console.log(arr3[arr3.length - 1]);
// or
console.log(arr3.slice(-1)[0]);
// `at` makes this a bit easier
console.log(arr3.at(-1));

// `at` also work on string
console.log('jonas'.at(0));
console.log('jonas'.at(-1));

Looping Arrays: forEach

const movements = [200, 450, -400, 3000, -650, -130, 70, 1300];

for (const [i, movement] of movements.entries()) {
  if (movement > 0) {
    console.log(`Movement ${i + 1}: You deposited ${movement}`);
  } else {
    console.log(`Movement ${i + 1}: You withdrew ${Math.abs(movement)}`);
  }
}

console.log('---- FOREACH ----');
// `continue` and `break` doesn't work in `forEach`
movements.forEach(function (mov, i, arr) {
  if (mov > 0) {
    console.log(`Movement ${i + 1}: You deposited ${mov}`);
  } else {
    console.log(`Movement ${i + 1}: You withdrew ${Math.abs(mov)}`);
  }
});

forEach with Map and Set

// Map
const currencies = new Map([
  ['USD', 'United States dollar'],
  ['EUR', 'Euro'],
  ['GBP', 'Pound sterling'],
]);

currencies.forEach(function (value, key, map) {
  console.log(`${key}: ${value}`);
});

// Set
const currenciesUnique = new Set(['USD', 'GBP', 'USD', 'EUR', 'EUR']);
console.log(currenciesUnique);
currenciesUnique.forEach(function (value, _, map) {
  // set doesnt have key
  console.log(`${value}: ${value}`);
});