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

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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

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Convert array to upper or lower case using ↴

map() method → creates a new array from calling a function for every array element.

toLowerCase() method → returns the value of the string converted to lower case.

toUpperCase() method → returns the value of the string converted to upper case.

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map() method creates a new array populated with the results of calling a provided function on every element in the calling array. The original array is unchanged.

const arr2 = [5, 10, 15, 20];

arr2.map((x) => x + 10); returns ↴

[15, 20, 25, 30] → 10 added to each element

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toLowerCase() method returns a new string with all letters converted to lower case. The original string is unchanged.

const str3 = "hELlo wORLd";

str3.toLowerCase(); returns ↴

"hello world" → lower case

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toUpperCase() method returns a new string with all letters converted to upper case. The original string is unchanged.

const str4 = "hELlo wORLd";

str4.toUpperCase(); returns ↴

"HELLO WORLD" → uppercase

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Initialize an array to convert case to lower or upper case.

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Define a function convertArrCase() to convert array elements to lower or upper case.

function convertArrCase(arr) {}

The function takes an array as input arr and returns a new array with the elements converted to lower or upper case. The original array is unchanged.

map() method creates a new array by applying a callback function to each element of the array.

map(calbackFn) ↴

callback function ↴

(word) => word.toLowerCase()

word → current element being processed in the array

The map method iterates over each character, converting it to lower case.

The result is a new array containing the lowercase versions of the original string.

To convert array elements to upper case ↴

toLowerCase() method is replaced with toUpperCase() method.

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Call the function with ↴

convertArrCase(array1);

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Convert array elements to lower case.

Convert array elements to upper case.