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

The question refers to incompliant behaviour in earlier ES6 implementations, notably V8 (Node.js 4 and legacy Chrome versions). The problem doesn't exist in modern ES6 implementations, both in strict and sloppy modes. const reassignment should always result in TypeError, it can be caught with try..catch.

There can't be isConstant function because const variable cannot be identified as such by its value.

It's preferable to run a script in strict mode and thus avoid problems that are specific to sloppy mode.

Even if a variable was defined in sloppy mode, it's possible to enable strict mode in nested function scope:

const foo = 1;
// ...
let isConst = false;

(() => {
  'use strict';

  try {
    const oldValue = foo;
    foo = 'new value';
    foo = oldValue;
  } catch (err) {
     isConst = true;
  }
})();

It's beneficial to use UPPERCASE_CONSTANT convention which is used in JavaScript and other languages. It allows to unambiguously identify a variable as a constant without aid from IDE and avoid most problems with accidental reassignments.