What Is It

Javascript by default moves all declarations to the top of the current scope

/* valid code, hoisted automatically */
x = 3; // assignment
... // other code
var x; // declaration

/* also a valid code, no need to hoist, identical to above */
var x; // declaration
x = 3; // assignment

Hoisting Behavior of let and const

Variables defined with let and const are hoisted, but not initialized

Using let before declaration is a ReferenceError

/* ReferenceError */
name = "Sam";
let name;

Using const before declaration is a syntax error

/* Invalid Syntax */ 
name = "Gil";
const name;