How to check that ES6 “variable” is constant?

Question

Does anyone know some tricks how to do it? I tried to use try-catch:

\"use strict\";

const a = 20;

var isConst = false;
try {
   var temp = a;         


        

Answer 1

Based on some of the answers here I wrote this code snippet (for client side JS) that will tell you how a "variable" was last declared--I hope it's useful.

Use the following to find out what x was last declared as (uncomment the declarations of x to test it):

// x = 0
// var x = 0
// let x = 0
// const x = 0

const varName = "x"    
console.log(`Declaration of ${varName} was...`)
try {
  eval(`${varName}`)
  try {
    eval(`var ${varName}`);
    console.log("... last made with var")
  } catch (error) {
    try {
      eval(`${varName} = 0`)
      console.log("... last made with let")
    } catch (error) {
      console.log("... last made with const")
    }
  }
} catch (error) {
  console.log("... not found. Undeclared.")
}

Interestingly, declaring without var, let or const, i.e x = 0, results in var getting used by default. Also, function arguments are re-declared in the function scope using var.