javascript - Are variables declared with let or const hoisted?

Question

I have been playing with ES6 for a while and I noticed that while variables declared with var are hoisted as expected...

console.log(typeof name); // undefined
var name = "John";

...variables declared with let or const seem to have some problems with hoisting:

console.log(typeof name); // ReferenceError
let name = "John";

and

console.log(typeof name); // ReferenceError
const name = "John";

Does this mean that variables declared with let or const are not hoisted? What is really going on here? Is there any difference between let and const in this matter?

Answer 1

@thefourtheye is correct in saying that these variables cannot be accessed before they are declared. However, it's a bit more complicated than that.

Are variables declared with let or const not hoisted? What is really going on here?

All declarations (var, let, const, function, function*, class) are "hoisted" in JavaScript. This means that if a name is declared in a scope, in that scope the identifier will always reference that particular variable:

x = "global";
// function scope:
(function() {
    x; // not "global"

    var/let/… x;
}());
// block scope (not for `var`s):
{
    x; // not "global"

    let/const/… x;
}

This is true both for function and block scopes¹.

The difference between var/function/function* declarations and let/const/class declara­tions is the initialisation.
The former are initialised with undefined or the (generator) function right when the binding is created at the top of the scope. The lexically declared variables however stay uninitialised. This means that a ReferenceError exception is thrown when you try to access it. It will only get initialised when the let/const/class statement is evaluated, everything before (above) that is called the temporal dead zone.

x = y = "global";
(function() {
    x; // undefined
    y; // Reference error: y is not defined

    var x = "local";
    let y = "local";
}());

Notice that a let y; statement initialises the variable with undefined like let y = undefined; would have.

The temporal dead zone is not a syntactic location, but rather the time between the variable (scope) creation and the initialisation. It's not an error to reference the variable in code above the declaration as long as that code is not executed (e.g. a function body or simply dead code), and it will throw an exception if you access the variable before the initialisation even if the accessing code is below the declaration (e.g. in a hoisted function declaration that is called too early).

Is there any difference between let and const in this matter?

No, they work the same as far as hoisting is regarded. The only difference between them is that a constant must be and can only be assigned in the initialiser part of the declaration (const one = 1;, both const one; and later reassignments like one = 2 are invalid).

_{1: var declarations are still working only on the function level, of course}