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

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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

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Count number of vowels found in a string using ↴

for loop → executes a block of code a number of times.

toLowerCase() method → returns the value of the string converted to lower case.

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for loop repeatedly executes a block of code until a specified condition evaluates to false.

The loop runs a block of code a set number of times, defined by an initialization, a condition, and an increment.

for (let x = 0; x < 4; x++) {

console.log(x);

}

Loop variable x is initialized to 0

Condition x < 4 is checked before each iteration.

The loop will continue to run as long as x is less than 4

The loop repeatedly executes a block of code 4 times, from 0 to 3

For each iteration of the loop, the current value of x is printed to the console.

After each iteration, x is incremented by 1 x++

When x reaches 4 the condition evaluates to false, terminating the loop.

0

1

2

3 → printed to console

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toLowerCase() method converts all letters to lower case. The original string is unchanged.

const str3 = "hELlo wORLd";

str3.toLowerCase(); returns ↴

"hello world" → lower case

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Initialize a string to count the number of vowels.

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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 lower case.

This ensures the search will be case in-sensitive and both lower case and upper case vowels will be found.

Initialize count of vowels to zero.

let count = 0 count

Loop through lowerStr

Get the current character lowerStr[x]

const char = lowerStr[x] char

Check if current character char matches a vowel character.

char === "a" || → OR

char === "e" || → OR

char === "i" || → OR

char === "o" || → OR

char === "u"

If a vowel is found increment count by 1

count++

Once the loop has completed return the count.

return count

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Call the function with ↴

countVowels(string1);

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Count number of vowels found in a string.