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
Remove duplicates from an array 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 an array from which to remove duplicates.
const array1 = [1, 2, 1, 2, 4, 3, 2, 1]; → user input
Define a function removeDuplicates to return a new array with all duplicates removed.
function removeDuplicates(arr) {}
The function takes an array as input arr and returns a new array with all duplicate values removed.
Use reduce() method to iterate over the array and remove duplicates.
const uniqueArr = arr.reduce() uniqueArr
reduce() method iterates through the array and executes a reducer function for each array element.
reduce(callbackFn, initialValue)
reduce((acc, curr) => (), [])
uniqueArr holds the result of the reduce operation
acc is the accumulator (initialized as an empty array)
curr is the current element
[] initialValue is an empty array
callback function ↴
(acc, curr) => {
if (acc.indexOf(curr) < 0) {
acc.push(curr)
}
return acc
}
initial value ↴
[] empty array
The reduce method executes a reducer function, resulting in a single output value, the acc array.
Check if current value curr is not already in the accumulator acc
if (acc.indexOf(curr) < 0)
indexOf returns -1 if curr cannot be found.
If -1 push curr into acc
acc.push(curr)
Return the accumulator acc for the next iteration.
return acc
Return the array with duplicates removed.
return uniqueArr
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()
Call the function with ↴
removeDuplicates(array1);
Remove duplicates from array.
reduce() method iterates over the array and constructs a new array without duplicates.
arr = [1, 2, 1, 2, 4, 3, 2, 1];
If the result of indexOf(curr) is -1 then curr is not in the accumulated array acc
Add curr to acc array ↴
acc.push(curr) push(curr)
If the result of indexOf(curr) is NOT -1 then curr is already in the accumulated array acc and can be ignored.
Iteration ↴
1 indexOf(1) -1 push(1) [1]
2 indexOf(2) -1 push(2) [1, 2]
3 indexOf(1) 0 [1, 2] already in array
4 indexOf(2) 1 [1, 2] already in array
5 indexOf(4) -1 push(4) [1, 2, 4]
6 indexOf(3) -1 push(3) [1, 2, 4, 3]
7 indexOf(2) 1 [1, 2, 4, 3] already in array
8 indexOf(1) 0 [1, 2, 4, 3] already in array
returns ↴
[1, 2, 4, 3] NO duplicate values. All values are unique.
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()
Remove duplicates from array.
const arr = [1, 2, 1, 2, 4, 3, 2, 1];
function removeDuplicates(arr) {
const uniqueArr = arr.reduce((acc, curr) => {
if (acc.indexOf(curr) < 0) {
acc.push(curr);
}
return acc;
}, []);
return uniqueArr;
}
call function
removeDuplicates(arr); returns ↴
[1, 2, 4, 3]