Scope Issue in Browser Dev Tool when Debugging Javascript Code

Question

I try deep diving into javascript behind the scenes and I am stuck at some point while debugging on Dev Tool on Browsers. Here is the story:

I have declare a block scope and inside that scope I have put one function declaration and a value. Since I am able to access that value inside that function, I couldn't understand why I can't see that value in the scope of the browser dev tool when my debugger inside of the function. Is there any idea about that?

{
  const a = 2;
  function abc() {
    console.log();
  }
  abc();
}

When the debugger inside of that function, the scope contains local and global scope. I expected there is also a block scope which contains variable I declared. However, when I write the code like that, I can see a block scope also when the debugger in that function.

{
  const a = 2;
  function abc() {
    console.log(a);
  }
  abc();
}

Answer 1

This behavior you're observing in the browser's debugger is related to how JavaScript engines optimize the handling of variables and scopes during execution.

1. In your first example, where abc() does not reference a, the JavaScript engine "may" optimize the scope handling. Since a is not used inside abc(), the engine might decide not to include the block scope in the scope chain of abc(). This is why you don't see the block scope when you pause inside abc() in the debugger.

2. In your second example, you're referencing a inside abc(). This creates a closure where abc() retains access to its outer scope, which includes the block where a is defined. Because of this reference, the JavaScript engine includes the block scope in the scope chain for abc(). That's why, when the debugger is paused inside abc(), you can see a.

JavaScript engines perform various optimizations at runtime, especially related to scope and variable handling, to improve performance. This can sometimes result in different behaviors in the debugger, especially if certain variables or scopes are not directly used or referenced within a function. These optimizations are generally not something developers need to worry about. However, they can be confusing while debugging.