Can't set values on `process.env` in client-side Javascript

Question

I have a system (it happens to be Gatsby, but I don't believe that's relevant to this question) which is using webpack DefinePlugin to attach some EnvironmentVariables to the global variable: process.env

I can read this just fine.

Unfortunatley, due to weirdnesses in the app startup proces, I need have chosen to do some brief overwritting of those EnvironmentVariables after the site loads. (Not interested in discussing whether that's the best option, in the context of this question. I know there are other options; I want to know whether this is possible)

But it doesn't work :(

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If I try to do it explicitly:

process.env.myVar = 'foo';

Then I get ReferenceError: invalid assignment left-hand side.

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If I do it by indexer (which appears to be what dotenv does) then it doesn't error, but also doesn't work:

console.log(process.env.myVar);
process.env['myVar'] = 'foo';
console.log(process.env.myVar);

will log undefined twice.

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What am I doing wrong, and how do I fix this?

Answer 1

The premise behind this attempted solution was flawed.

I was under the impression that webpack "made process.env.* available as an object in the browser".

It doesn't!

What it actually does is to transpile you code down into literals wherever you reference process.env. So what looks like fetch(process.env.MY_URL_VAR); isn't in fact referencing a variable, it's actually being transpiled down into fetch("http://theActualValue.com") at compile time.

That means that it's conceptually impossible to modify the values on the "process.env object", because there is not in fact an actual object, in the transpiled javascript.

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This explains why the direct assignment gives a ref error (you tried to execute "someString" = "someOtherString";) but the indexer doesn't. (I assume that process.env gets compiled into some different literal, which technically supports an indexed setter)

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The only solutions available would be to modify the webpack build process (not an option, though I will shortly raise a PR to make it possible :) ), use a different process for getting the Env.Vars into the frontEnd (sub-optimal for various other reasons) or to hack around with various bits of environment control that Gatsby provides to make it all kinda-sorta work (distasteful for yet other reasons).