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 words in 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.
Regular Expression → patterns used to match character combinations in strings.
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
trim() method removes whitespace from both ends of a string and returns a new string, without modifying the original string.
const arr4 = " Hello World "; → string with leading and trailing whitespace
arr4.trim(); returns ↴
"Hello World" → string without leading or trailing whitespace
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
Regular expressions are patterns used to match character combinations in strings.
The Regular Expression /\s+/ matches one or more whitespace characters.
\s matches any whitespace character (spaces, tabs, line breaks).
+ quantifier indicates that one or more occurrences of the preceding element (whitespace in this case) should be matched.
As a result, this regex will match any sequence of whitespace characters, effectively splitting the string at each space.
This means that multiple spaces between words will not create empty entries in the resulting array.
split(/\s+/) will split the string into an array of words.
const str5 = "A storm in a teacup"; → string
str5.split(/\s+/); returns ↴
["A", "storm", "in", "a", "teacup"] → array
Initialize a variable to hold the string to count number of words.
const string1 = "The quick brown fox jumps over the lazy dog"; → user input
Define a function countWords to count number of words in a string.
function countWords(str) {}
The function takes a string as input, str and returns the number of words found in that string.
Trim whitespace from both ends of the string and split it into an array of words.
return str
Remove leading and trailing whitespace.
.trim()
Split the string into an array of words based on one or more whitespace characters.
.split(/\s+/)
Use the reduce() method to count the number of words in the array.
reduce() method iterates over the array of words and returns a single value, the count of words.
reduce(callbackFn, initialValue) ↴
reduce((count) => count + 1, 0)
count is the accumulator (initialized as 0)
The currentValue parameter is not explicitly used within the function body.
0 initialValue is zero
callback function ↴
(count) => count + 1
The reduce method starts with an initial count of 0 and increments the count by 1 for each word found.
initial value ↴
0 zero
The return value becomes the value of the accumulator parameter count 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 ↴
countWords(string1);
Count number of words in a string.
const string1 = "The quick brown fox jumps over the lazy dog";
function countWords(str) {
return str
.trim()
.split(/\s+/)
.reduce((count) => count + 1, 0);
}
call function
countWords(string1); returns ↴
9