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 ↴

forEach() method → calls a function for each element in an array, executing a provided function once for each 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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forEach() method calls a function for each element in an array, executing a provided function once for each array element.

The method does not return a new array, it always returns undefined

const arr3 = [2, 4, 6, 8];

arr3.forEach((element, index) => {

console.log(index, element)

}); returns ↴

0 2

1 4

2 6

3 8 → index and element printed to console

Use forEach() when an action is needed to be performed on each element,

not when a new array needs to be generated from the current one.

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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 the array elements converted to lower or upper case. The original arrays is updated.

forEach() method loops through the array, executing a callback function once for each array element.

arr.forEach(callbackFn) ↴

callback function ↴

The callback function takes three parameters: element, index, and array.

element → current element being processed

index → index of the current element

array → original array being modified

toLowerCase() method is called on each element, and the result is assigned back to the corresponding index in the original array.

Return the modified array.

return arr

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.