How Do Hoisting, the Temporal Dead Zone, and Initialization Differ Between `var`, `let`, and `const` in JavaScript?

Variable Hoisting with Let and Const

While it's true that variables declared with var are hoisted, the behavior of variables declared with let and const in this regard can be confusing. To resolve this confusion, let's break down what's happening.

Hoisting: A Universal Concept in JavaScript

All variable declarations in JavaScript, including var, let, const, functions, and class declarations, are hoisted. Essentially, the identifier is made available within the scope it's declared.

x = "global";
(function() {
    x; // not "global"
    {
        x; // not "global"
        let y; // not initialized
    }
    var x = "local";
    let y = "local";
});

In this example, all variable declarations are hoisted within their respective scopes (function and block).

Temporal Dead Zone: An Exception for Let and Const

The distinction between var declarations and let/const declarations lies in the initialization. var and other old-style declarations are initialized with undefined or the function object when the binding is created at the top of the scope. In contrast, let and const declarations remain uninitialized until the statement is executed.

This creates what's known as the temporal dead zone - a period between the variable's creation and its initialization. Attempting to access the variable within this zone results in a ReferenceError exception.

x = y = "global";
(function() {
    x; // undefined
    y; // ReferenceError: y is not defined
    var x = "local";
    let y = "local";
});
// Block where temporal dead zone applies

No Difference Between Let and Const in Hoisting

Both let and const declarations follow the same hoisting behavior. The only difference is that const variables must be initialized at the time of declaration and cannot be reassigned later.

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