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Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase, before the code is executed. This means that regardless of where variables and functions are declared within their scope, they are hoisted (i.e., conceptually moved) to the top of the scope.

There are two main aspects of hoisting in JavaScript:

1. Variable Hoisting:
Variables declared with `var` are hoisted to the top of their containing function or global scope.
However, only the variable declaration is hoisted, not the initialization. This means that the variable is initialized with the value `undefined` until the actual assignment is encountered during execution.

2. Function Hoisting:
Function declarations are also hoisted to the top of their containing scope.
Unlike variables, both the function declaration and definition are hoisted, which means you can call the function before it is declared in the code.

It's important to note that hoisting only applies to declarations, not initializations or assignments. Additionally, hoisting behavior differs between variables declared with `var`, `let`, and `const`. Variables declared with `let` and `const` are hoisted to the top of their containing block but are not initialized until their declaration statement is reached. This results in a "temporal dead zone" (TDZ) where accessing the variable before its declaration throws a ReferenceError.

Understanding hoisting is crucial for writing predictable and maintainable JavaScript code, as it can affect the behavior of your code if not properly understood and accounted for.

Scope Chain:
The scope chain refers to the hierarchy of variable scopes in JavaScript. When a variable is referenced, JavaScript looks for it first in the current scope, then in the outer scopes, traversing up the scope chain until it finds the variable or reaches the global scope. This mechanism allows functions to access variables from their parent scopes through closure.