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
Objects are a data structure used to store related data collections.
It stores data as key-value pairs, where each key is a unique identifier for the associated value.
Each key must be a string and must be unique, each value can be any data type.
If you define an object with duplicate keys, the last one will overwrite any preceding ones.
Find the value for any given key in the object.
const obj1 = {"A": 4, "B": 5, "C": 6 }; object
obj1["A"]; key "A" → 4
obj1["B"]; key "B" → 5
obj1["C"]; key "C" → 6
obj1["D"]; key "D" → undefined key not found
Find the value for any given key in the object.
const obj2 = {"A": 4, "B": 5, "C": 6 }; object
const str = "ABC"; string
obj2[str[0]]; → 4
obj2[str[1]]; → 5
obj2[str[2]]; → 6
obj2[str[3]]; → undefined key not found
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
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 frequency of each element in an array using the reduce() method.
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
Initialize an array to count frequency of each element.
const array1 = ["A", "C", "B", "A", "D", "A", "B", "C", "B", "A"]; → user input
Define a function frequency to count the frequency of each element in the array.
function frequency(arr) {}
The function takes an array as input arr and returns an object representing the frequency of each element. The original array remains unchanged.
The keys represent the unique elements from the array.
The values denote the number of times each element appears.
Use the reduce() method to count the frequency of each element in an array.
reduce() method iterates through the array and executes a reducer function for each array element, transforming the array into an object that maps each unique element to its frequency count.
reduce(callbackFn, initialValue)
return arr.reduce((count, element) => {})
count is the accumulator (initialized as an empty object)
element is the current element
{} initialValue is an empty object
callback function ↴
Check if the element is already a key in the count object.
if (!count[element])
If it is NOT, add the element to the count object and assign it the value 0
count[element] = 0
else if element already exists, increment the count by 1
count[element]++
Return object containing the frequency of each element.
return count
initial value ↴
{} empty object
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()
keys represent the unique elements in the array.
values denote the number of times each element appears.
["a", "b", "a", "c", "b", "a"] → array
{a: 3, b: 2, c:1} → object
"a" found 3 times
"b" found 2 times
"c" found 1 time
Call the function with ↴
frequency(array1);
Count frequency of each element in array.
const array1 = ["A", "C", "B", "A", "D", "A", "B", "C", "B", "A"];
function frequency(arr) {
return arr.reduce((count, element) => {
if (!count[element]) {
count[element] = 0;
}
count[element]++;
return count;
}, {});
}
call function
frequency(array1); returns ↴
{A: 4, C: 2, B: 3, D: 1}
Alternative - using a ternaray operator ↴
const array2 = ["A", "C", "B", "A", "D", "A", "B", "C", "B", "A"];
function frequency2 (arr2) {
const freq = arr2.reduce((count, element) => {
return count[element] ? ++count[element]: count[element] = 1, count;
}, {});
return freq;
}
call function
frequency2 (array2) returns ↴
{A: 4, C: 2, B: 3, D: 1}