In JavaScript, what will happen if you try to access a variable declared with let before it is initialized?

When a variable is declared using the let keyword in JavaScript, it is hoisted to the top of its block scope. However, unlike variables declared with var, which are initialized to undefined, let declarations are not initialized until the actual statement of the variable is executed. Therefore, if you attempt to access a let variable before its declaration and initialization, JavaScript throws a ReferenceError. This error signifies that you are trying to access a variable that is in the "temporal dead zone" — the time period between entering the block scope and the let declaration being encountered.

This behavior is an important aspect of block scoping in JavaScript, helping to avoid potential bugs that could arise from undefined variables. In contrast, trying to access an undefined variable declared with var would simply return undefined, which is different from the strict limitation imposed by let. As such, understanding how hoisting and scoping work in JavaScript is crucial for writing error-free code.