Essential Javascript / Basic data types
let v;
console.log(v); // -> undefined
v = 5;
console.log(v); // -> 5
v = "str";
console.log(v); // -> strINFO
let v;
console.log(v); // -> undefined
v = 5;
console.log(v); // -> 5
v = "str";
console.log(v); // -> str