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.
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 ↴
const arr2 = [1, 2, 3, 4, 5, 6];
arr2.reduce((acc, cur) => acc + cur, 0); Initial value → 0
returns ↴
21
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 ↴
const arr3 = [11, 12, 13, 14, 15, 16, 17];
const evenNumbers = arr3.reduce((acc, cur) => {
return cur % 2 === 0 ? [...acc, cur] : acc;
}, []); Initial value → []
console.log(evenNumbers); returns ↴
[12, 14, 16]
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.
Iteration ↴
0 acc 11 → [] Initial value is an empty array
1 acc 11 → []
2 acc 12 → [12] 12 added to acc array
3 acc 13 → [12]
4 acc 14 → [12, 14] 14 added to acc array
5 acc 15 → [12, 14]
6 acc 16 → [12, 14, 16] 16 added to acc array
7 acc 17 → [12, 14, 16]
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.
("") separator → string is split between each character.
(" ") separator → string is split at each space character, resulting in an array of words.
const str3 = "Hello"; → string
str3.split(""); returns ↴
["H", "e", "l", "l", "o"] → array
const str4 = "hello world"; → string
str4.split(" "); returns ↴
["hello", "world"] → array
Initialize a variable to hold the string to be reversed.
const string1 = "Hello World"; → user input
Define a function reverseString to reverse a string.
function reverseString(str) {}
The function takes a string as input str and returns a new string with the characters reversed. The original string is unchanged.
Split the string str into an array of characters.
split("")
Use reduce() method to iterate over the array and reverse the string.
return str.split("").reduce()
reduce() method iterates through the array and returns a single string with the characters reversed.
reduce(callbackFn, initialValue)
reduce((rev, char) => (), "")
rev is the accumulator (initialized as an empty string)
char is the current element
"" initialValue is an empty string
callback function ↴
(rev, char) => char + rev
initial value ↴
"" empty string
The reduce method executes a reducer function, resulting in a single output value, reversed string.
Concatenate current char with the accumulated rev
(rev, char) => char + rev, ""
The return value becomes the value of the accumulator parameter rev 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 ↴
reverseString(string1)
Reverse a string.
str = "Hello"
During each iteration, the current character char will be prepended to rev at the start of the string ↴
Iteration ↴
0 rev Initial value → ""
1 char "H" + rev "H"
2 char "e" + rev "eH"
3 char "l" + rev "leH"
4 char "l" + rev "lleH"
5 char "o" + rev "olleH"
The return value becomes the value of the accumulator parameter rev on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
returns ↴
"olleH" reversed string
Reverse a string.
const string1 = "Hello World";
function reverseString(str) {
return str.split("").reduce((rev, char) => char + rev, "");
}
call function
reverseString(string1); returns ↴
"dlroW olleH"
Alternatives to convert a string into an array of substrings ↴
const str6 = "Hello";
str6.split(""); returns ↴
["H", "e", "l", "l", "o"] → array
const str7 = "Hello";
[...str7]; returns ↴
["H", "e", "l", "l", "o"] → array
const str8 = "Hello";
Array.from(str8); returns ↴
["H", "e", "l", "l", "o"] → array