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
Calculate sum of all numbers using ↴
reduce() method → executes a reducer function, resulting in a single output value.
length property → sets or returns the number of elements in an 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 is an empty array []
console.log(evenNumbers); returns ↴
[12, 14, 16]
During each interation, the ternary operator evaluates the condition cur % 2 === 0
If current element cur is divisible by 2, it will be added to the accumulator [...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
1 acc 11 → []
2 acc 12 → [12] 12 added
3 acc 13 → [12]
4 acc 14 → [12, 14] 14 added
5 acc 15 → [12, 14]
6 acc 16 → [12, 14, 16] 16 added
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
length property returns the number of elements in an array.
const arr4 = [1, 2, 3, 4, 5, 6];
arr4.length; returns ↴
6 → there are 6 elements in the array
Initialize a variable to hold the array to calculate the average.
const array1 = [2, 4, 6, 8, 10]; → user input
Define a function findAvgerage to calculate the average of an array.
function findAvgerage(arr) {}
The function takes an array of numbers as input arr and returns the average of that array.
Use the reduce method to calculate the sum of the array.
reduce() method iterates over the array of numbers.
reduce(callbackFn, initialValue)
arr.reduce((sum, num) => sum + num, 0)
sum → accumulator, initialized to 0
num → current element
0 → initialValue
callback function ↴
((sum, num) => sum + num, 0)
The reduce method is used to sum up the elements of the array.
const summedArr = arr.reduce((sum, num) => sum + num, 0) summedArr
The return value becomes the value of the accumulator parameter sum on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
Return the average by dividing summedArr by the number of elements in the array arr
return summedArr / arr.length
Call the function with ↴
findAvgerage(array1);
Calculate the average of an array.
const array1 = [2, 4, 6, 8, 10];
function findAverage(arr) {
const summedArr = arr.reduce((sum, num) => sum + num, 0);
return summedArr / arr.length;
}
call function
findAverage(array1); returns ↴
6
Alternative - return on one line ↴
return arr.reduce((sum, num) => sum + num, 0) / arr.length;