https://youtu.be/MyPOSFaiaDU

arrays.js

// Arrays

let arr = new Array();
let arr = []; // mostly used

// Declare array with some initial data:
let fruits = ["Apple", "Orange", "Plum"];

// GET element from array - use index
let fruits = ["Apple", "Orange", "Plum"];

let apple = fruits[0]; // Apple
let orange = fruits[1]; // Orange
let plum = fruits[2]; // Plum

// Index is a number starts from 0
//  --0--     --1--     --2--
// ["Apple", "Orange", "Plum"]

// UPDATE elements in array - use index
fruits[2] = "Pear"; // now ["Apple", "Orange", "Pear"]

// ADD new element to array - use index
fruits[3] = "Lemon"; // now ["Apple", "Orange", "Pear", "Lemon"]

// Arrays can contain ANY types inside, even mix of many types:
let arr = [
    "Apple",
    { name: "John" },
    true,
    function() {
        alert("hello");
    }
];

// get the object at index 1 and then show its name
let n = arr[1]; // John
// get the function at index 3 and run it
arr[3](); // hello

// use isArray to check if your variable is array:
let arr = [1, 2, 3, 4, 5];
console.log(Array.isArray(arr)); // true

// ***************************************************************************
// pop/push, shift/unshift

// pop - take away last element
let fruits = ["Apple", "Orange", "Pear"];
console.log(fruits.pop()); // remove "Pear" and print it
console.log(fruits); // Apple, Orange

// push - add element to the end of array
let fruits = ["Apple", "Orange"];
fruits.push("Pear");
console.log(fruits); // Apple, Orange, Pear

// shift - take away first element
let fruits = ["Apple", "Orange", "Pear"];
console.log(fruits.shift()); // remove Apple and print it
console.log(fruits); // Orange, Pear

// unshift - add element to begining
let fruits = ["Orange", "Pear"];
fruits.unshift("Apple");
console.log(fruits); // Apple, Orange, Pear

// You can add multiple elements at once
let fruits = ["Apple"];
fruits.push("Orange", "Peach");
fruits.unshift("Pineapple", "Lemon");
// ["Pineapple", "Lemon", "Apple", "Orange", "Peach"]
console.log(fruits);

// ***************************************************************************
// Arrays are Objects:

let fruits = []; // make an array
fruits[100] = 5; // assign a property with the index far greater than its length
fruits.age = 25; // create a property with an arbitrary name

// Special property - length
// Represents number of items in array:
let fruits = [];
console.log(fruits.length); // 0
fruits.push("a", "b", "c");
console.log(fruits.length); // 3

// Do not be confused with length\index!
console.log(fruits[2]); // "c"

// UNSAFE!
// You can modify value of length:
let fruits = [];
console.log(fruits.length); // 0
fruits.push("a", "b", "c");
console.log(fruits.length); // 3
fruits.length = 1000;
console.log(fruits.length); // 1000
fruits.push("d");
console.log(fruits.length); // 1001

// ***************************************************************************
// Iterating (loops) for array

let fruits = ["Apple", "Orange", "Pear"];

// (1) Just regular 'for' loop with checking length
for (let i = 0; i < fruits.length; i++) {
    console.log(fruits[i]);
}

// (2) for/of - special form for iterables(arrays)
let fruits = ["Apple", "Orange", "Plum"];
// for..of doesn’t give access to the index of the current element
for (let fruit of fruits) {
    console.log(fruit);
}

// (!) Do NOT use for/in loop (as for iterating over object properties)
let arr = ["Apple", "Orange", "Pear"];
for (let key in arr) {
    console.log(arr[key]); // Apple, Orange, Pear
}
// because for..in iterates over ALL properties, not just indexes
arr["some"] = "oops!";
for (let key in arr) {
    console.log(arr[key]); // Apple, Orange, Pear, oops!
}

// (3) forEach method, runs provided function for every member of array
let arr = ["Apple", "Orange", "Pear"];
arr.forEach(function(item, index, array) {
    console.log(item);
});
// The result of the function (if it returns any) is thrown away and ignored.
let res = arr.forEach(function(item, index, array) {
    return "NEW" + item;
});
console.log(arr)
console.log(res)

// (4) .map() method
let arr = [10, 30, 100];
let newArr = arr.map(function(item, index, array) {
    console.log(item);
    return item + 10;
});
console.log(newArr);
// The result of the function will be passed to new array

arrays_other.js

// ***************************************************************************
// Other array methods

// ***************************************************************************
// splice

// arr.splice(index[, deleteCount, elem1, ..., elemN])
// starts from the position index:
// removes deleteCount elements
// and then inserts elem1, ..., elemN at their place.
// Returns the array of removed elements.

let arr = ["I", "study", "JavaScript"];
arr.splice(1, 1); // from index 1 remove 1 element
console.log(arr); // ["I", "JavaScript"]

// **
let arr = ["I", "study", "JavaScript", "right", "now"];
// remove 3 first elements and replace them with another
arr.splice(0, 3, "Let's", "dance");
console.log(arr); // now ["Let's", "dance", "right", "now"]

// **
let arr = ["I", "study", "JavaScript", "right", "now"];
// remove 2 first elements
let removed = arr.splice(0, 2);
console.log(removed); // "I", "study"

// **
let arr = ["I", "study", "JavaScript"];
// from index 2
// delete 0 elements
// then insert "complex" and "language"
arr.splice(2, 0, "complex", "language");
console.log(arr); // "I", "study", "complex", "language", "JavaScript"

// ***************************************************************************
// slice
// Returns a new array containing all items from index "start" to "end"
let arr = ["a", "b", "c", "d"];
console.log(arr.slice(1, 3)); // [b,c]
console.log(arr.slice(-2)); // [c,d]
console.log(arr);

// ***************************************************************************
// concat
// Merges 2 arrays

let arr = [1, 2];

// merge arr with [3,4]
console.log(arr.concat([3, 4])); // 1,2,3,4

// merge arr with [3,4] and [5,6]
console.log(arr.concat([3, 4], [5, 6])); // 1,2,3,4,5,6

// merge arr with [3,4], then add values 5 and 6
console.log(arr.concat([3, 4], 5, 6)); // 1,2,3,4,5,6

// ***************************************************************************
// sort

let arr = [1, 2, 15];
// the method reorders the content of arr (and returns it)
arr.sort();
console.log(arr); // 1, 15, 2

// UNSAFE!
// Why it is sorted to - 1, 15, 2 ??????
// The items are sorted as strings by default.
console.log("2" > "15"); // true

// To avoid this - supply your own sorting function:

function compareNumeric(a, b) {
  if (a > b) return 1;
  if (a == b) return 0;
  if (a < b) return -1;
}
let arr = [1, 2, 15];
arr.sort(compareNumeric);
console.log(arr); // 1, 2, 15

// comparison function should return a positive number to say “greater”
// and a negative number to say “less”.
// 0 - "same"
// So you can write the same:
arr.sort((a, b) => a - b);

// ***************************************************************************
// reverse

let arr = [1, 2, 3, 4, 5];
arr.reverse();
console.log(arr); // 5,4,3,2,1

// ***************************************************************************
// split and join

// split can be used to split string to array:
let names = 'one, two, three';
let arr = names.split(', ');
console.log(arr) // [ 'one' 'two' 'three' ] 


// join - will join array items to string:
let str = arr.join(';');
console.log(str)

arrays_search.js

// searching
let fruits = ["Apple", "Orange", "Plum"];
// looks for item starting from index from, and returns the index where it was found, otherwise -1.
let index = fruits.indexOf("Plum", 1);

// same, but looks from right to left.
let index = fruits.lastIndexOf("Plum");



// includes returns true if found.
let isExist = fruits.includes("Orange");
console.log(isExist); // true
// Note that the methods use === comparison

// ***************************************************************************
// find and findIndex
// Returns first item for which function returned 'true', or undefined
let users = [
    { id: 1, name: "John", age: 21 },
    { id: 2, name: "Pete", age: 43 },
    { id: 3, name: "Mary", age: 37 }
];

let user = users.find(function(item) {
    return item.id == 1;
});

console.log(user.name); // John

// findIndex works the same but returns index of element, or -1 if not found
let ind = users.findIndex(function(item) {
    return item.id == 2;
});

console.log(ind); // 1

// some - to check if atleast one element in array matches condition
let hasUserLessThan18 = users.some(function(user) {
    return user.age < 18;
});

console.log(hasUserLessThan18); // false

// every - to check that ALL elements in array matches condition
let allUsersMoreThan18 = users.every(function(user) {
    return user.age > 18;
});

console.log(allUsersMoreThan18); // true

// ***************************************************************************
// filter
let users = [
    { id: 1, name: "John", age: 43 }, // same age
    { id: 2, name: "Pete", age: 43 },
    { id: 3, name: "Mary", age: 37 }
];

// returns new array that will contain only items for which function returned 'true'
let usersWithAge43 = users.filter(function(item) {
    return item.age == 43
});

console.log(usersWithAge43.length); // 2
console.log(usersWithAge43) 
/*
[ { id: 1, name: 'John', age: 43 }, 
  { id: 2, name: 'Pete', age: 43 } ] 
*/