CODEWITHSUNDEEP
javascriptscopeconstants
Tree Nguyen
Tree Nguyen

Reputation: 1199

How to declare CONST when it doesn't exist in Javascript

So in short, I have a module that I want to reuse multiple time on a page. In this module, I had a global const of an event name. The problem occurs at the declaration of the second usage of this module since the constant has already been declared. However, I can't move it into if/else since scoping will make the constant unable to be used elsewhere. What should I do?

Some simple code for my problem:

Module:

const THIS_IS_MY_EVENT = 'thisIsMyEvent';

// extra code down here

Page: (I use twig engine to include file but I don't think it's related)

<script src="/path/to/file.js">
<script src="/path/to/file.js">

This sure won't work in my case:

if (typeof (THIS_IS_MY_EVENT) == undefined) {
    const .... //this will limit the scope of my const
}

Any help would be appreciated.

Upvotes: 5

Views: 2637

Answers (1)

T.J. Crowder
T.J. Crowder

Reputation: 1074138

So in short, I have a module that I want to reuse multiple time on a page.

That sounds like the real problem to solve. Instead, import the module once, use a function within it twice. Modules generally shouldn't do anything on import (other than a bit of setup), only later when requested. That way, the global parts of the module will run once, creating the global const (but it shouldn't be a global, it should be an exported const from the module), and you won't have the problem.

But conditionally creating a global constant is an interesting problem. If you really want a global, you can use Object.defineProperty to create it on window with writable: false. (This isn't quite the same. Properties on window [the global object] are globals, but const creates globals that aren't properties of window. Still, probably close enough.)

Example:

"use strict";
if (typeof THIS_DOESNT_EXIST === "undefined") {
  Object.defineProperty(window, "THIS_DOESNT_EXIST", {
    value: "foo",
    writable: false,     // false is the default
    configurable: false, // these are just for emphasis,
    enumerable: true     // or false, your call
  });
}
console.log(THIS_DOESNT_EXIST);
THIS_DOESNT_EXIST = 42;
console.log(THIS_DOESNT_EXIST);

Upvotes: 4

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