What will be the output of console.log(x); let x = 10;?

When you attempt to log the value of x to the console before it has been initialized, you will encounter a ReferenceError. This occurs because let declarations are block-scoped and are not hoisted in the same way that var declarations are. While var would allow you to access the variable and receive undefined, let does not allow access to the variable until the declaration has been encountered in the code.

In this case, console.log(x) is executed before the line let x = 10; is reached, leading to a situation where x remains uninitialized and unreferenced. As a result, JavaScript raises a ReferenceError, indicating that x is not defined at the point of the console log. This behavior is part of the temporal dead zone concept in JavaScript, which prevents access to variables before they are declared.