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 union of two arrays using ↴
reduce() method → executes a reducer function, resulting in a single output value.
concat() method → merge two or more arrays.
includes() method → returns true if an array contains a specified value, otherwise returns false.
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
concat() method is used to merge two or more arrays.
This method does not change the existing arrays, but instead returns a new array.
const arr4 = [1, 2, 3, 4, 5];
const arr5 = [4, 5, 6, 7, 8];
const arr6 = arr4.concat(arr5);
console.log(arr6); returns ↴
[1, 2, 3, 4, 5, 4, 5, 6, 7, 8] → values are not unique
includes() method determines whether an array includes a certain value among its entries, returning true or false
const arr7 = [1, 2, 3, 4, 5, 6];
arr7.includes(4); returns boolean ↴
true → 4 found in array
const arr8 = [1, 2, 3, 4, 5, 6];
arr8.includes(7); returns boolean ↴
false → 7 NOT found in array
logical NOT ! syntax converts a true value to a false and vice-versa.
!arr8.includes(7); returns boolean ↴
true → 7 NOT found in array
push() method adds new elements to the end of an array.
Add 4 to end of array.
const arr9 = [1, 2, 3];
arr9.push(4);
console.log(arr9); returns ↴
[1, 2, 3, 4] → 4 added to end of array
The push() method changes the length of the array.
arr9 is modified.
Using the spread operator creates a new array.
Add 4 to a new array.
const arr10 = [1, 2, 3];
const arr11 = [...arr10, 4];
console.log(arr11); returns ↴
[1, 2, 3, 4] → 4 added to new array
console.log(arr10); returns ↴
[1, 2 ,3]
arr10 remains unchanged.
Initialize the two input arrays to find their union.
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 findUnion() to find the union of two arrays.
function findUnion(arr1, arr2) {}
The function takes two arrays as input arr1, arr2 and returns a new array with their union. The original arrays remain unchanged.
Each input array may contain duplicates. Duplicates will be removed inside the function.
concat() method ↴
arr1.concat(arr2) method merges the two input arrays arr1, arr2, into a new array.
Use reduce() method to combine the elements of both arrays into a single array without duplicates.
return arr1.concat(arr2).reduce()
reduce() method iterates over the contatenated arrays and returns an array of unique values.
reduce(callbackFn, initialValue) ↴
reduce((union, element) => (), [])
union is the accumulator (initialized as an empty array)
element is the current element
[] initialValue is an empty array
calback function ↴
(union, element) => {
if (!union.includes(element)) {
union.push(element)
}
return union
}
initial value ↴
[] empty array
The reduce method executes a reducer function, resulting in a single output value, the union array.
includes() method checks whether the current element is among the entries of the union array.
!union.includes(element)
If it is NOT found, the element will be included in the union array.
push() method adds the element to the union array if it is NOT found.
union.push(element)
The return value becomes the value of the accumulator parameter union on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
The function returns the union array, which contains all the unique elements from both input arrays.
If there is no union between the two arrays then an empty array is returned.
Concatenate both input arrays into a new single array.
return arr1.concat(arr2)
reduce the new array to a unique set of values.
.reduce((union, element) => {
Check if the current element is not already included in the union array.
if (!union.includes(element)) {
If not, push it into the union array.
union.push(element)
Return the updated union array.
return union
Initialize the union array as an empty array.
}, [])
Call the function with ↴
findUnion(array1, array2);
Find union of two arrays.
const array1 = [1, 2, 3, 4, 5];
const array2 = [4, 5, 6, 7, 8];
function findUnion(arr1, arr2) {
return arr1.concat(arr2).reduce((union, element) => {
if (!union.includes(element)) {
union.push(element);
}
return union;
}, []);
}
call function
findUnion(array1, array2); returns ↴
[1, 2, 3, 4, 5, 6, 7, 8]
Alternative ↴
Instead of includes() use indexOf() method.
!union.includes(element) ↴
union.indexOf(element) === -1
Alternative ↴
Instead of the concat() method use the spread syntax ... to merge arrays.
arr1.concat(arr2) ↴
[...arr1, ...arr2]