Const is not defined in global scope but defined in local?

Question

Why const is not defined in global scope but defined in local? screenshoot from devTools

{
    console.log(b);
    const b = 2;
}
VM366:2 Uncaught ReferenceError: Cannot access 'b' before initialization
    at <anonymous>:2:17

console.log(a);
const a = 1;
VM382:1 Uncaught ReferenceError: a is not defined
    at <anonymous>:1:13

Answer 1

This seems to be related to some "magic" in how REPL input is handled. It's not really a property of how JavaScript normally works.

If you try this in a node.js file, in a <script>, or even in an (0, eval)('...'), you'll get "Cannot access 'a' before initialization" as expected. Note that even in the DevTools console this won't happen if you are in a paused debugger state.

In a non-paused state (only there), the DevTools allow you to declare variables as if you were continuously writing a script, and the declarations will persist across commands. There is clearly "magic" outside of regular JavaScript involved, because even eval would create another temporary scope. That "magic" is handled by V8 itself. It is therefore pretty likely that whatever trickery is used here has the side effect of this slightly unexpected error. (I initially thought the side effect is that the variable declaration isn't hoisted, but if you do a = 0; const a = 1 then you get "Assignment to constant variable", so that doesn't seem to be the case.)

I tried to trace this to the source, but I got lost in the depths of V8. Nonetheless, I discovered (by opening another DevTools to debug the DevTools themselves!) that this only happens if the replMode argument (that gets passed to the V8 backend's evaluate method) is true. This can be verified by setting a breakpoint here and changing the value of options.replMode.

replMode is documented as follows:

Setting this flag to true enables let re-declaration and top-level await. Note that let variables can only be re-declared if they originate from replMode themselves.

Answer 2

• Hoisting workd differently for const and let than var.
• SO for const and let, which are block scoped gives ReferenceError error as they do not initialize value as undefined. -Whereas, for var it will initialize value as undefinded if you do not assign any v alue and var is function scoped. -You can see console from devToolis giving different error, whereasconsolefromstackoverflow``` giving error as our expectation that

"Uncaught ReferenceError: Cannot access 'a' before initialization. So I believe it's how devtool console treating the error for block-scope and global-scope. -Check it on ```morzila-devtool`.

{
  console.log("before initialization:" + b);
  var b = 2;
  console.log("after initialization:" + b);
}
console.log("before initialization:" + a);
const a = 1;

• This is from Firefox browser

• This is from Microsoft Edge

• I came to know from this post that, Chrome and IE both have a global event object. So when we are trying to access it first hoisting won't work for const and let. so it's says that we cannot access but window object has access to those variables, So I assume it's throwing error for a is not defined for window object, which is global object.

• window object sets automatically to undefind. const window.variable = undefined (when not defined); Try to run this code:

{console.log(b); var b=1; console.log(window.c); const c=1;}
OUTPUT:
1 debugger eval code:1:9
undefined // window.c = undefined

And as window object has access to global scope it gives error for a or b or variable is not defined when we access it in global scope before initialisation using let or const.