7.5Objects and Arrays

const fruits = ['mango', 'apple', 'banana'];
fruits[0];                 // 'mango'
fruits.length;             // 3
fruits[1] = 'orange';      // change an item

fruits.push('kiwi');       // add to end
fruits.pop();              // remove from end
fruits.unshift('lime');    // add to start
fruits.shift();            // remove from start
fruits.indexOf('banana');  // position, or -1
fruits.includes('apple');  // true or false
fruits.slice(1, 3);        // copy a part
fruits.splice(1, 1);       // remove 1 item at index 1
fruits.join(', ');         // 'mango, banana'
fruits.sort();             // sorts alphabetically (in place)
fruits.reverse();

// Higher-order methods: very commonly used
const nums = [1, 2, 3, 4, 5];
nums.forEach(n => console.log(n));                 // do something for each
const doubled = nums.map(n => n * 2);              // [2,4,6,8,10]
const evens   = nums.filter(n => n % 2 === 0);     // [2,4]
const total   = nums.reduce((sum, n) => sum + n, 0); // 15
const found   = nums.find(n => n > 3);             // 4

// 2D array
const grid = [[1,2],[3,4]];
grid[1][0];                // 3

const student = {
  name: 'Kamal',
  age: 21,
  courses: ['HTML', 'JS'],
  address: { city: 'Negombo', zip: '11500' },
  greet() {
    return 'Hi, I am ' + this.name;       // this = the object
  }
};

student.name;                 // dot notation
student['age'];               // bracket notation (use when key is in a variable)
student.address.city;         // nested
student.email = '[email protected]';    // add property
delete student.age;           // remove property
student.greet();              // call a method

Object.keys(student);         // array of keys
Object.values(student);       // array of values
Object.entries(student);      // array of [key, value]

const { name, courses } = student;     // destructuring
const copy = { ...student, age: 22 };  // spread: copy with changes

const products = [
  { id: 1, name: 'Pen',  price: 50 },
  { id: 2, name: 'Book', price: 250 }
];
const cheap = products.filter(p => p.price < 100);
const names = products.map(p => p.name);
const totalPrice = products.reduce((s, p) => s + p.price, 0);