JavaScript: Arrays

Creating arrays

// Literal
const tags = ['html', 'css', 'js'];

// From values
const numbers = Array.of(1, 2, 3);

// From iterable / array-like
const letters = Array.from('abc'); // ['a','b','c']

Common methods

• push/pop: add/remove end
• unshift/shift: add/remove start
• map, filter, reduce: transform
• slice (no mutate) vs splice (mutate)
• includes, indexOf, find

const nums = [1, 2, 3];
nums.push(4);          // [1,2,3,4]
const doubled = nums.map(n => n * 2); // [2,4,6,8]
const evens = nums.filter(n => n % 2 === 0); // [2,4]

Iteration

const list = ['a','b','c'];

// Index-based
for (let i = 0; i < list.length; i++) {
  console.log(i, list[i]);
}

// Values
for (const value of list) {
  console.log(value);
}

// forEach
list.forEach((v, i) => console.log(i, v));

Immutable patterns

const a = [1, 2, 3];

// Add without mutating 'a'
const b = [...a, 4];       // [1,2,3,4]
const c = a.concat(4, 5);  // [1,2,3,4,5]

// Remove without mutating
const withoutFirst = a.slice(1); // [2,3]

Practice

1. Create an array of 5 numbers; build a new array with each number squared.
2. From ['css','html','js'], return only items containing j.
3. Add an item to the end without mutating the original.

Next up

• Objects