Before ES6, variable declarations were only possible using var. With ES6, we got let and const. Both let and const declarations are block-scoped, i.e., they can only be accessed within the " { } " surrounding them. On the other hand, var doesn't have such a restriction.
Unlike var, which can be accessed before its declaration, you cannot access the let or const variables until they are initialized with some value. Temporal Dead Zone is the time from the beginning of the execution of a block in which let or const variables are declared until these variables are initialized. If anyone tries to access those variables during that zone, Javascript will always throw a reference error as given below.
console.log(varNumber); // undefined
console.log(letNumber); // Throws the reference error letNumber is not defined
var varNumber = 3;
let letNumber = 4;
Both let and const variables are in the TDZ from the moment their enclosing scope starts to the moment they are declared.
