Strings are a sequence of zero or more characters written inside quotes used to represent text.

Strings may consist of letters, numbers, symbols, words, or sentences.

Strings are immutable, they cannot be changed.

Each character in a string has an index.

The first character will be index 0 the second character will be index 1 and so on.

There are two ways to access an individual character in a string.

charAt() method

const str1 = "abc"; string

str1.charAt(0); character at index 0 → "a"

str1.charAt(1); character at index 1 → "b"

str1.charAt(2); character at index 2 → "c"

str1.charAt(3); character at index 3 → "" index not found

Alternatively use at() or slice() methods

bracket notation []

const str2 = "abc"; string

str2[0]; character at index 0 → "a"

str2[1]; character at index 1 → "b"

str2[2]; character at index 2 → "c"

str2[3]; character at index 3 → undefined index not found

---

Arrays are used to store multiple values in a single variable.

Each value is called an element, and each element has a numeric position in the array, known as its index.

Arrays are zero-indexed, meaning the first element is at index 0, the second at index 1, and so on.

Arrays can contain any data type, including numbers, strings, and objects.

const arr1 = [2, 4, 6]; array

arr1[0]; element at index 0 → 2

arr1[1]; element at index 1 → 4

arr1[2]; element at index 2 → 6

arr1[3]; element at index 3 → undefined index not found

---

Numbers are used to represent both integer and floating-point values.

Numbers are most commonly expressed in literal forms like 255 or 3.14159 ↴

let num1 = 5; → number

let num2 = 2.5; → number

let num3 = num1 + num2;

console.log(num3); returns ↴

7.5 → number

---

Count number of vowels found in a string using ↴

Array.from() method → creates a new array instance from an array-like or iterable object.

filter() method → creates a new array filled with elements that pass a test provided by a function.

includes() method → returns true if an array contains a specified value, otherwise returns false.

length property → sets or returns the number of elements in an array.

---

Array.from() method creates a new array instance from an array-like or iterable object, such as a string. The original string is unchanged.

const str3 = "Hello"; string

Array.from(str3); returns ↴

["H", "e", "l", "l", "o"] → array

---

includes() method determines whether an array includes a certain value among its entries, returning true or false

const arr2 = [1, 2, 3, 4, 5, 6];

arr2.includes(4); returns boolean ↴

true → 4 found in array

const arr3 = [1, 2, 3, 4, 5, 6];

arr3.includes(7); returns boolean ↴

false → 7 NOT found in array

---

length property returns the number of elements in an array.

const arr4 = [1, 2, 3, 4, 5, 6];

arr4.length; returns ↴

6 → there are 6 elements in the array

---

Initialize a string to count the number of vowels.

---

Initialize a variable to hold the lower case and upper case vowels.

Define a function countVowels to count number of vowels found in a string

function countVowels(str) {}

The function takes a string as input, str and returns the number of vowel characters found in that string.

Convert str to an array of characters to iterate over each character.

Array.from(str)

filter() method creates a new array with all elements that pass the test implemented by the callback function.

filter(callbackFn) ↴

callback function ↴

(char) => vowels.includes(char)

char → current element being processed in the array

includes() method checks whether the current element is among the entries of the vowels string.

If it is, that character is kept in the new array.

Return the length of the array.

.length

---

Call the function with ↴

countVowels(string1);

---

Count number of vowels found in a string.

---

Alternatives to convert string to an array ↴

Object.assign([], str);

Array.from(str);

str.split("");

[...str];