Hoisting is a term used in JavaScript to describe the behavior of variable and function declarations being moved to the top of their respective scopes during the compilation phase of code execution.
In other words, JavaScript treats variable and function declarations differently from other statements, such as assignments or function expressions. When the JavaScript engine encounters a variable or function declaration, it moves it to the top of the current scope (global or function), regardless of where the actual declaration appears in the code.
This means that a variable or function can be used in the code before it has been declared. However, only the declaration is hoisted, not the initialization or assignment. If a variable is declared but not assigned a value, its value will be undefined.
Here is an example of hoisting in action:
console.log(x); // undefined
var x = 5;
In this example, the variable x is declared and initialized with a value of 5. However, when the console.log statement is executed, x has not yet been assigned a value and its value is undefined. This happens because the declaration of x is hoisted to the top of the scope, but the assignment is not.
It's important to note that only variable and function declarations are hoisted in JavaScript, not other types of statements like let, const, or class. Additionally, hoisting can sometimes lead to unexpected behavior and should be used with caution. It's generally recommended to declare and initialize variables at the top of their respective scopes to avoid confusion and bugs.