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.
reduce() method → executes a reducer function, resulting in a single output value.
indexOf() method → returns the first index at which a given element can be found 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
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
indexOf() method returns the first index at which a given element can be found in an array, or -1 if it is not present.
Find first index of element "C" in the array.
const arr4 = ["A", "B", "C", "A", "B", "C"];
arr4.indexOf("C"); returns ↴
2 → index of first occurrence of element "C"
Element is present.
Find first index of element "D" in the array.
const arr5 = ["A", "B", "C", "A", "B", "C"];
arr5.indexOf("D"); returns ↴
-1 → element "D" NOT found in array,
Element NOT present.
array.indexOf(element) !== -1
If result is NOT -1 then the element is present in the array.
Ternary Operator
Ternary Operator is frequently used as an alternative to if...else statements.
if (condition) {
expressionIfTrue;
} else {
expressionIfFalse;
}
Ternary Operator takes three operands ↴
condition evaluates to true or false.
? question mark → expression to execute if the condition is truthy
: colon → expression to execute if the condition is falsy
condition ? expressionIfTrue : expressionIfFalse
let score = 75;
let result = score >= 55
? "You passed"
: "You failed";
console.log(result); returns ↴
You passed → first expression executed
Initialize a string to count the number of vowels.
const string1 = "HELLO World"; → user input
Initialize a variable to hold the lower case and upper case vowels.
const vowels = "aeiouAEIOU" 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)
Use the reduce() method to count number of vowel characters found.
return Array.from(str).reduce()
reduce() method iterates through the array and executes a reducer function for each array element.
reduce(callbackFn, initialValue) ↴
reduce((count, char) => (), 0)
count is the accumulator (initialized to 0)
char is the current character
0 initialValue is zero
callback function ↴
(count, char) => (vowels.indexOf(char) !== -1 ? count + 1 : count)
The callback function checks if the current character char exists in the vowels string.
vowels.indexOf(char) !== -1
A ternary operator evaluates a condition to either true or false.
If current character char exists in the vowels string it will evaluate to true
Increment the accumulated count by 1
? count + 1
If current character char does NOT exist in the vowels string it will evaluate to false
Return count unchanged.
: count
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 ↴
countVowels(string1);
Count number of vowels found in a string.
const string1 = "HELLO World";
const vowels = "aeiouAEIOU";
function countVowels(str) {
return Array.from(str).reduce(
(count, char) => (vowels.indexOf(char) !== -1 ? count + 1 : count),
0
);
}
call function
countVowels(string1); returns ↴
3 → vowels found "E" "O" "o"
Alternatives to convert string to an array ↴
Object.assign([], str)
Array.from(str)
str.split("")
[...str]