let can be updated but not re-declared.

Just like var,  a variable declared with let can be updated within its scope. Unlike var, a let variable cannot be re-declared within its scope. So while this will work:

let greeting = "say Hi";
greeting = "say Hello instead";

this will return an error:

let greeting = "say Hi";
let greeting = "say Hello instead"; // error: Identifier 'greeting' has already been declared

However, if the same variable is defined in different scopes, there will be no error:

let greeting = "say Hi";
if (true) {
    let greeting = "say Hello instead";
    console.log(greeting); // "say Hello instead"
}
console.log(greeting); // "say Hi"

Why is there no error? This is because both instances are treated as different variables since they have different scopes.

This fact makes let a better choice than var. When using let, you don't have to bother if you have used a name for a variable before as a variable exists only within its scope.

Also, since a variable cannot be declared more than once within a scope, then the problem discussed earlier that occurs with var does not happen.