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
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
Find the difference between two arrays using ↴
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.
push() method → adds specified elements to the end of an array and returns the new length of the 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 6 in the array.
const arr4 = [2, 4, 6, 8, 6, 4];
arr4.indexOf(6); returns ↴
2 → index of first occurrence of element 6
Element is present.
Find first index of element 9 in the array.
const arr5 = [2, 4, 6, 8, 6, 4];
arr5.indexOf(9); returns ↴
-1 → element 9 NOT found in array.
Element NOT present.
array.indexOf(element) !== -1
If result is NOT -1 then the element is present in the array.
push() method adds new elements to the end of an array.
Add 4 to end of array.
const arr6 = [1, 2, 3];
arr6.push(4);
console.log(arr6); returns ↴
[1, 2, 3, 4] → 4 added to end of array
The push() method changes the length of the array.
arr6 is modified.
Using the spread operator creates a new array.
Add 4 to a new array.
const arr7 = [1, 2 , 3];
const arr8 = [...arr7, 4];
console.log(arr8); returns ↴
[1, 2, 3, 4] → 4 added to new array
console.log(arr7); returns ↴
[1, 2 ,3]
arr7 remains unchanged.
Initialize the two input arrays to find their difference.
Each input array should contain unique elements with no duplicates.
first array ↴
const array1 = [1, 2, 3, 4, 5]; → user input
second array ↴
const array2 = [4, 5, 6, 7, 8]; → user input
Define a function findDifference() to find the difference between two arrays.
function findDifference(arr1, arr2) {}
The function takes two arrays as input arr1, arr2 and returns a new array with their difference. The original arrays remain unchanged.
Each input array should contain unique elements with no duplicates.
Use the reduce() method to iterate over the first array arr1 and build the difference array.
return arr1.reduce()
reduce() method iterates over the array and returns a single array with the elements that appear in one array but not the other.
reduce(callbackFn, initialValue) ↴
reduce((difference, element) => (), [])
difference is the accumulator (initialized as an empty array)
element is the current element
[] initialValue is an empty array
callback function ↴
(difference, element) => {
if (arr2.indexOf(element) === -1) {
difference.push(element)
}
return difference
}
The reduce method executes a reducer function, resulting in a single output value, the difference array.
indexOf() method returns the first index at which a given element can be found in the array.
arr2.indexOf(element) === -1
If the result of indexOf() is -1 then the current element is NOT found in arr2, the element is unique to arr1
push() method adds the element to the difference array if it is NOT found.
difference.push(element)
After the loop completes, the difference array is returned.
return difference
initial value ↴
[] empty array
The return value becomes the value of the accumulator parameter difference on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
The function returns a new array containing elements that are present in the first array but not in the second array.
If there is no difference between the two arrays then an empty array is returned.
Alternatively ↴
reduce() method iterates through the second array, arr2, checking for elements that are NOT present in the first array, arr1
Find all elements that are in the first array arr1 but not in the second array arr2
Use reduce to accumulate unique elements.
return arr1.reduce((difference, element) => {}
Check if the current element is NOT in arr2
if (arr2.indexOf(element) === -1) {
If NOT found, push it to the difference array.
difference.push(element)
Return the accumulated difference array
return difference
Initialize the accumulator as an empty array.
}, [])
Find all elements that are in the second array arr2 but not in the first array arr1
Use reduce to accumulate unique elements.
return arr2.reduce((difference, element) => {}
Check if the current element is NOT in arr1
if (arr1.indexOf(element) === -1) {
If NOT found, push it to the difference array.
difference.push(element)
Return the accumulated difference array
return difference
Initialize the accumulator as an empty array.
}, [])
Call the function with ↴
findDifference(array1, array2);
Find elements in first array that are NOT in the second array.
Find difference between two arrays.
const array1 = [1, 2, 3, 4, 5];
const array2 = [4, 5, 6, 7, 8];
function findDifference(arr1, arr2) {
return arr1.reduce((difference, element) => {
if (arr2.indexOf(element) === -1) {
difference.push(element);
}
return difference;
}, []);
}
call function
findDifference(array1, array2); returns ↴
[1, 2, 3]
Find elements in second array that are NOT in the first array.
Find difference between two arrays.
const array1 = [1, 2, 3, 4, 5];
const array2 = [4, 5, 6, 7, 8];
function findDifference(arr1, arr2) {
return arr2.reduce((difference, element) => {
if (arr1.indexOf(element) === -1) {
difference.push(element);
}
return difference;
}, []);
}
call function
findDifference(array1, array2); returns ↴
[6, 7, 8]
The input to the function should not contain any duplicate values.
Set Object is a collection of unique values where each value can only occur once, each value is unique.
Remove duplicates from array ↴
const myArr1 = [1, 2, 3, 4, 1, 2, 3, 4];
const setA = new Set(myArr1); set
const myArr2 = [...setA]; spread syntax
console.log(myArr2); returns ↴[1, 2, 3, 4] → array
same as ↴
const myArr3 = [1, 2, 3, 4, 1, 2, 3, 4];
[...new Set(myArr3)]; returns ↴
[1, 2, 3, 4] → array
Alternative ↴
Instead of indexOf() use includes() method.
array.indexOf(element) === -1
!array.includes(element)