Let keyword - JavaScript Tutorial - w3Schools - Ch#08 English

The let keyword was introduced in ES6 (2015).
Variables defined with let cannot be Redeclared.
Variables defined with let must be Declared before use.
Variables defined with let have Block Scope.

Variables defined with let are also hoisted to the top of the block, but not initialized. Meaning: Using a let variable before it is declared will result in a ReferenceError.