why we have to declare a variable in javascript code example

Example 1: javascript declare a variable

//choose the best for your solution
var myVariable = 22; //this can be a string or number. var is globally defined

let myVariable = 22; //this can be a string or number. let is block scoped

const myVariable = 22; //this can be a string or number. const is block scoped and can't be reassigned

Example 2: how to make a variable equal a specific element in javascript

//How to make a variable equal a specific element in javascript.

/*====DESCRIPTION STARTS HERE=======
Assuming you've already created an array and a variable, you simply set the
variable equal to the array, specifying the specific location of the value
it holds. Note though that when specifying the location, subtract one
number from the actual number we percieve. The reason for this is that computers
begin counting at 0.
  =====DESCRIPTION ENDS HERE========*/

//=====EXAMPLE=========
var listOfFirstNames = ["Will", "Kotori", "Grace", "James"]
var FirstName = listOfFirstNames[2] /*firstName now has the value of "Grace"
									since grace is the 2nd (or 3rd) element
									of the array*/