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 ↴
recursion → programming technique where a function calls itself repeatedly to solve a problem.
slice() method → returns selected elements in an array, as a new array.
length property → set or return the number of elements in an array.
Recursion The act of a function calling itself.
Recursion is used to solve problems that contain smaller sub-problems.
A recursive function can receive two inputs: a base case (ends recursion) or a recursive case (resumes recursion).
Use recursion to find the factorial of 5.
let x = 5;
function factorial(num) {
if (num > 1) { Recursion call
return num * factorial(num - 1);
}
else { Base case
return 1;
}
}
call function
factorial(x); returns ↴
120 factorial of 5 → 120
slice() method returns selected elements in an array, as a new array.
syntax ↴
slice(start) return a new array from start index to end of array
slice(start, end) return an array from start index to end index of array (exclusive).
Return a new array from index 1 to end of array.
const arr2 = [1, 2, 3, 4];
arr2.slice(1); start index is 1
returns ↴
[2, 3, 4]
Return a new array from index 1 to index 4 (exclusive).
const arr3 = [1, 2, 3, 4];
arr3.slice(1, 3); start index is 1 end index is 4 (not included)
returns ↴
[2, 3]
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 sum of all numbers.
const array1 = [2, 4, 6, 8, 10]; → user input
Define a function findSum to calculate the sum of all numbers in an array.
function findSum(arr) {}
The function takes an array as input arr and returns the sum of all numbers in that array.
Base case
If the array is empty, return 0 and end execution of function.
if (arr.length === 0) return 0
Recursive case
If the array is not empty, the function returns the first element of the array arr[0] added to the result of calling findSum on the rest of the array arr.slice(1)
The slice method creates a new array that excludes the first element.
return arr[0] + findSum(arr.slice(1))
Call the function with ↴
findSum(array1);
Calculate sum of all numbers in an array.
const array1 = [2, 4, 6, 8, 10];
function findSum(arr) {
if (arr.length === 0) return 0;
return arr[0] + findSum(arr.slice(1));
}
call function
findSum(array1); returns ↴
30