[JavaScript] Arrays

These are my notes while doing the course JavaScript Algorithms and Data Structures on https://www.freecodecamp.org. I highly recommend it if you prefer to try things directly rather than watching videos.

Note: In JavaScript arrays are technically a type of object. Therefore arrays are also capable of storing complex objects.

Array functions to manipulate arrays:

/* Modify Array Data with Indexes */
var myArray = [50,40,30];
myArray[0] = 15; // equals [15,40,30]


/* Access Multi-Dimensional Arrays with Indexes */
var myArray = [
  [1,2,3],
  [4,5,6],
  [7,8,9],
  [[10,11,12], 13, 14]
];
myArray[3]; // equals [[10,11,12], 13, 14]
myArray[3][0]; // equals [10,11,12]
myArray[3][0][1]; // equals 11


/* Manipulate Arrays with push() -> adding to the end */
var myArray = [1,2,3];
myArray.push(4); // myArray is now [1,2,3,4]


/* Manipulate Arrays with pop() -> removing last element */
var threeArr = [1, 4, 6];
var oneDown = threeArr.pop();
console.log(oneDown); // Returns 6
console.log(threeArr); // Returns [1, 4]


/* Manipulate Arrays with shift() -> removing first element */
var myArray = ["Stimpson", "J", ["cat"]];
var removedFromOurArray = myArray.shift();
console.log(removedFromOurArray); // Returns "Stimpson"
console.log(myArray); // Returns ["J", ["cat"]]


/* Manipulate Arrays with unshift() -> adding to the beginning */
var myArray = ["Stimpson", "J", "cat"];
myArray.shift(); // myArray now equals ["J", "cat"]
myArray.unshift("Happy"); // myArray now equals ["Happy", "J", "cat"]


/* Manipulate Arrays with concat() -> combine arrays into a new one without mutating the original arrays. */
var firstArray = [1, 2, 3];
var secondArray = [4, 5, 6];
var thirdArray = firstArray.concat(secondArray); // thirdArray now equals [ 1, 2, 3, 4, 5, 6 ]
/* Also useable to clone an array (will do a copy by reference!) */
var fourthArray = [].concat(firstArray); // FourthArray now equals [1, 2, 3]


/* Remove & Add Items using splice() -> remove any number of consecutive elements from anywhere in an array. */
/* First parameter: index */
/* Second parameter: number of elements to delete */
let array = ['today', 'was', 'not', 'so', 'great'];
array.splice(2, 2);  // array now equals ['today', 'was', 'great']
/* Third parameter: add elements to the array. Useful for quickly switching out an element, or a set of elements, for another. */
const numbers = [10, 11, 12, 12, 15];
numbers.splice(3, 1, 13, 14); // returns [ 10, 11, 12, 13, 14, 15 ]

Array functions that does not mutate the original array:

/* Copy Array Items Using slice() -> copies or extracts a given number of elements to a new array, leaving the array it is called upon untouched */
let weatherConditions = ['rain', 'snow', 'sleet', 'hail', 'clear'];
let todaysWeather = weatherConditions.slice(1, 3); // todaysWeather equals ['snow', 'sleet'];


/* Copy an Array with the Spread Operator -> allows to easily copy all of an array's elements. Syntax: ... */
let thisArray = [true, true, undefined, false, null];
let thatArray = [...thisArray]; // thatArray equals [true, true, undefined, false, null]


/* Combine Arrays with the Spread Operator */
let fragment = ['to', 'code'];
let sentence = ['learning', ...fragment, 'is', 'fun']; // sentence equals ['learning', 'to', 'code', 'is', 'fun']


/* The reduce() method reduces the array to a single value. It executes a provided function for each value of the array (from left-to-right).*/
/* First parameter: The initialValue, or the previously returned value of the function */
/* Second parameter: The value of the current element */
/* 0 is set as initialValue */
const args = [2, 4, 2];
const arraySum = args.reduce((sum, currentNum) => sum + currentNum, 0); // returns 8 


/* map() iterates over each item in an array and returns a new array containing the results of calling the callback function on each element */
const users = [
  { name: 'John', age: 34 },
  { name: 'Amy', age: 20 },
  { name: 'camperCat', age: 10 }
];
const names = users.map(user => user.name); // [ 'John', 'Amy', 'camperCat' ]

Checks:

/* indexOf() checks for the presence of an element -> takes element as parameter. Returns the position (index) or -1 if element does not exist */
let fruits = ['apples', 'pears', 'oranges', 'peaches', 'pears'];
fruits.indexOf('dates'); // returns -1
fruits.indexOf('oranges'); // returns 2
fruits.indexOf('pears'); // returns 1, the first index at which the element exists


/* every() checks if every element passes a particular test */
function checkPositive(arr) {
   return arr.every(num => num > 0); 
}
checkPositive([1, 2, 3, -4, 5]); // false


/* some() checks if any element passes a particular test. */
function checkPositive(arr) {
  return arr.some(num => num > 0); 
}
checkPositive([1, 2, 3, -4, 5]); // true

And there are several other build in methods like forEach(), filter(), etc.
https://www.w3schools.com/jsref/jsref_obj_array.asp

Note: As already mentioned, arrays are objects, therefore array functions are methods on the array object prototype, i.e. Array.prototype.push()