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

---

Reverse a string using ↴

reduce() method → executes a reducer function, resulting in a single output value.

split() method → splits a string into an array of substrings.

trim() method → removes whitespace from both ends of the string and returns a new string, without modifying the original string.

---

reduce() method iterates over each element in an array, and each iteration returns a single value, which is the accumulator.

When the iteration is finished, the accumulator value will be returned from the method.

It takes two parameters: a callback function and an optional initial value ↴

callback function first parameter.

initialValue second parameter. The accumulator is initialized to the first element of the array if no initial value is provided.

The callback function takes four parameters ↴

accumulator The value resulting from the previous call to callback function - required.

currentValue The value of the current element - required.

currentIndex Index position of currentValue in the array - optional.

Array The array reduce() was called upon - optional.

syntax

array.reduce(callback, initialValue); ↴

array.reduce((accumulator, currentValue, currentIndex, Array), initialValue)

---

Example 1 | Find the sum of the array ↴

During each iteration, the current value cur will be added to the accumulator acc ↴

acc + cur

Iteration ↴

0 acc Initial value → 0

1 acc 0 → 0 + 1 = 1 → 1

2 acc 1 → 1 + 2 = 3 → 3

3 acc 3 → 3 + 3 = 6 → 6

4 acc 6 → 6 + 4 = 10 → 10

5 acc 10 → 10 + 5 = 15 → 15

6 acc 15 → 15 + 6 = 21 → 21

The return value becomes the value of the accumulator parameter acc on the next invocation of the callback function.

For the last invocation, the return value becomes the return value of reduce()

When the iteration is finished, the accumulator value will be returned ↴

21 → sum of the array

---

Example 2 | Find even numbers ↴

During each iteration, the ternary operator evaluates the condition cur % 2 === 0

If current element cur is divisible by 2, it will be added to the accumulator array, acc

[...acc, cur] ↴

cur % 2 === 0 ? [...acc, cur] : acc

If current element cur not divisible by 2, the accumulator acc will be returned as is.

The return value becomes the value of the accumulator parameter acc on the next invocation of the callback function.

For the last invocation, the return value becomes the return value of reduce()

When the iteration is finished, the accumulator value will be returned ↴

[12, 14, 16] → even numbers

---

split() method splits a string into an array of substrings based on a specified separator (delimiter). The original string is unchanged.

const str3 = "Hello"; → string

str3.split(""); returns ↴

["H", "e", "l", "l", "o"] → array

const str4 = "Hello World"; → string

str4.split(" "); returns ↴

["Hello", "World"] → array

---

trim() method removes whitespace from both ends of a string and returns a new string, without modifying the original string.

---

Initialize a variable to hold the string to be reversed.

---

Define a function reverseWords to reverse a string of words.

function reverseWords(str) {}

The function takes a string as input str and returns a new string with the words in reverse order. The original string is unchanged.

Split the string str into an array of words based on spaces.

str.split(" ")

Use reduce() method to iterate over the array of words and reverse the order of words.

reduce() method iterates over the array of words and returns a single string, reversing the order of words.

reduce(callbackFn, initialValue)

reduce((reversed, word) => (), "")

reversed is the accumulator (initialized as an empty string)

word is the current element

"" initialValue is an empty string

For each word it concatenates it to the front of the reversed string, effectively reversing the order.

Initial value is an empty string ""

Remove any leading or trailing whitespace from the resulting string.

.trim()

The return value becomes the value of the accumulator parameter reversed on the next invocation of the callback function.

For the last invocation, the return value becomes the return value of reduce()

---

Call the function with ↴

reverseWords(string1);

---

Reverse order of words.

---

Alternative to remove one or more spaces from string using a regular expression as the separator to split the string into an array of words using one or more spaces.