let numbers = [1, 2, 4, 7];

console.log(numbers); //=> [ 1, 2, 4, 7 ]

let numbers = [1, 2, 4, 7];

console.log(numbers[0]); //=> 1
console.log(numbers[3]); //=> 7
console.log(numbers[4]); //=> undefined
console.log(numbers); //=> [ 1, 2, 4, 7 ]

let numbers = [1, 2, 4, 7];

// changing value
numbers[2] = 5;

// loading element
numbers[4] = 10;
console.log(numbers); //=> [ 1, 2, 5, 7, 10 ]

delete numbers[4];
console.log(numbers); //=> [ 1, 2, 5, 7,  ]

let numbers = [];

numbers[0] = 10;
numbers[1] = 20;
numbers[10] = 100;

console.log(numbers); //=> [ 10, 20, , , , , , , , , 100 ]

let values = [1, 'fulano', true, null, [1, 2]];

console.log(values[1]); //=> 'fulano'
console.log(values[4]); //=> [1, 2]
console.log(values[4][1]); //=> 2

let values = [];

values[0] = 1;
values['number'] = 10;
values[1] = 20;

console.log(values); //=> [ 1, 20, number: 10 ]

let students = [];

students[0] = [1, 'fulano', 'fulano@gmail.com'];
students[1] = [2, 'sicrano', 'sicrano@gmail.com'];

console.log(students[0][2]); //=> 'fulano@gmail.com'

console.log(students); //=> [ [ 1, 'fulano', 'fulano@gmail.com' ], [ 2, 'sicrano', 'sicrano@gmail.com' ] ]

let students = [];

students[0] = [];
students[0]['id'] = 1;
students[0]['name'] = 'Fulano';
students[0]['email'] = 'fulano@gmail.com';

console.log(students[0]['email']); //=> 'fulano@gmail.com'
console.log(students[0].email); //=> 'fulano@gmail.com'

console.log(students); //=> [ [ id: 1, name: 'Fulano', email: 'fulano@gmail.com' ] ]

let numbers = [1, 2, 3];

console.log([...numbers, 4, 5]); //=> [ 1, 2, 3, 4, 5 ]

// for
let numbers = [1, 2, 4, 7];
let result = '';

for (let flag = 0; flag < numbers.length; flag++) {
  result += numbers[flag] + ' ';
}

console.log(result); //=> '1 2 4 7 '

// for..in
let values = [];

values[0] = 1;
values['number'] = 10;
values[1] = 20;

for (let index in values) {
  console.log(`${index} => ${values[index]}`);
}
//=>
// '0 => 1'
// '1 => 20'
// 'number => 10'

// for..of
let values = [1, 20, 10];

for (let value of values) {
  console.log(value);
}
//=>
// '1'
// '20'
// '10'