Are Variables Declared With Let or Const Hoisted

Are variables declared with let or const hoisted?

@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}

Why can't assignment be done before the variable declaration with let and const?

Because they're explicitly designed not to allow that, because it's usually a programming mistake.

let and const are hoisted, but it's just the declaration of the binding that's hoisted. (Loosely, "binding" means "variable" [or constant or parameter...things with names we use to hold values].) The binding is not initialized until later, when the let or const statement is reached in the step-by-step execution of the code. You can't use an uninitialized binding (in any way), which is why you get an error.

In contrast, with var both declaration and initialization are hoisted; var bindings are initialized with the value undefined. If there's an initialization value on the var (var a = 42), later when the var statement is reached in the step-by-step execution of the code, that part is treated as simple assignment (a = 42). With let and const, it's not just simple assignment, it's initialization of the binding, allowing it to be used.

Here's a concrete example of how let hoists the declaration but not the initialization, and why it helps prevent programming mistakes:

let a = 1;

function foo() {
    a = 2;  // <=== Which `a` should be assigned to?
    console.log(a);
    
    // code
    // code
    // code
    // code
    // code
    // code
    // code
    // code

    let a = 3;
    console.log(a);
}

foo();

JavaScript: Difference in hoisting for let, const?

One thing that const or let difference from var is that the variable is not accessible before declaration.

Just checkout the specification: http://www.ecma-international.org/ecma-262/6.0/#sec-let-and-const-declarations

The variables are created when their containing Lexical Environment is instantiated but may not be accessed in any way until the variable’s LexicalBinding is evaluated.

Syntax

LexicalDeclaration_{[In, Yield]} :

    LetOrConst BindingList_{[?In, ?Yield]} ;

LetOrConst :

    let

    const

BindingList_{[In, Yield]} :

    LexicalBinding_{[?In, ?Yield]}

    BindingList_{[?In, ?Yield]} , LexicalBinding_{[?In, ?Yield]}

LexicalBinding_{[In, Yield]} :

    BindingIdentifier_[?Yield] Initializer_{[?In, ?Yield]opt}

    BindingPattern_[?Yield] Initializer_{[?In, ?Yield]}

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