How can I find out if an ES6 value is var / const / let after it is declared?

Question

I know I can find out if a value is var const or let by looking at where it is declared. However I am wondering - mainly for debugging, developing JS compilers, and academic interest - if it is possible to find out the immutability / scope of a variable (var / const / let-ness) after it has been created.

ie

doThing(something)

Would return

let

Or equivalent. Like we can determine types with typeof or something.constructor.name for constructors.

Answer 1

You can distinguish let and const from var with direct eval in the same function as the variable:

let a;

try {
    eval('var a');
    // it was undeclared or declared using var
} catch (error) {
    // it was let/const
}

As @JonasWilms had written earlier, you can distinguish let from const by attempting assignment:

{
  let x = 5;
  let isConst = false;
  
  try {
    x = x;
  } catch (err) {
    isConst = true;
  }
  
  console.log(isConst ? 'const x' : 'let x');
}

{
  const x = 5;
  let isConst = false;
  
  try {
    x = x;
  } catch (err) {
    isConst = true;
  }
  
  console.log(isConst ? 'const x' : 'let x');
}