how to handle variable which is not event declared in javascript code example

Example 1: js variables

// let & var are both editable variables:
var cow = 'moo';
let pig = 'oink';
cow = 10;
pig = 10;
// const is a permanent variable; you cannot change it.
const uncuttableGrass = '\/\/';
uncuttableGrass = '___';
// above throws an error

Example 2: Create variable javascript

//Assigns the value bar to the variable foo
var foo = bar;