In JavaScript, hoisting is a mechanism where variable declarations (var) and function declarations are moved to the top of their respective scopes during the compilation phase before the code is executed.

This means that even if you declare a variable or a function after you have used it, it will still be accessible because the declaration is hoisted to the top of the scope.

For example, consider the following code:

console.log(x);
var x = 10;

The output of the above code will be undefined, not an error. This is because the variable declaration is hoisted to the top of its scope, but its assignment is not. So when the console.log statement is executed, x has been declared, but its value is undefined.

Similarly, consider the following code:

foo();
function foo() {
    console.log("Hello world!");
}

The output of the above code will be Hello world!. This is because the function declaration is hoisted to the top of its scope, so it is available to be called before it is defined.

It's important to note that only the declaration is hoisted, not the initialization. This means that if you declare a variable but don't assign a value to it, its value will be undefined.

Hoisting can be a powerful tool in JavaScript, but it can also lead to confusion if you're not careful. It's important to understand how hoisting works so that you can write clear and understandable code.