React Native - .ENVs are undefined in Metro Bundler's config file when bundling iOS apps - ios

Question

I have NPM scripts that build and run our app with mocked backend data (used for component testing). They set ENVs inline using the cross-env npm package.
"android-mock": "cross-env MOCK_BACKEND=1 NODE_ENV=development node generate-config.js && react-native run-android -- --reset-cache", - emulator runs on Windows
"ios-mock": "cross-env MOCK_BACKEND=1 NODE_ENV=development node generate-config.js && react-native run-ios -- --reset-cache", - simulator runs on MacOS
Metro bundler looks for the MOCK_BACKEND env and based on that, it resolves paths so that the app uses mocked data instead of relying on the backend to get it.
Problem: Metro can read the process.env.MOCK_BACKEND value only when bundling Android apps, on iOS, the value returns undefined.
What is the correct way to define ENVs so that Metro can read them properly also for iOS bundling?

Answer 1

I don't know why, but it seems like Metro overwrites set ENVs when bundling iOS apps.
Elsewhere ENVs are defined.
If anyone comes over this problem, this is how I solved it.
Solution
1. In the generate-config.js script that is in NPM scrips above, I have added NODE_ENV checks. The script generates the config.js inside the public folder, based on NODE_ENV.
const fs = require("fs");
console.log( process.env.NODE_ENV);
if(process.env.NODE_ENV === 'production'){
fs.copyFile("config.runtime.js","public/config.js",(err) => {if(!err){console.log(err)}});
} else if (process.env.NODE_ENV === 'mock'){
fs.copyFile("config.mock.js","public/config.js",(err) => {if(!err){console.log(err)}});
} else {
fs.copyFile("config.debug.js","public/config.js",(err) =>{if(!err){console.log(err)}});
}
2. I have imported generated config.js into Metro.config and used it as needed.
const config = require(path.resolve(__dirname, 'public/config.js'));
config.js for reference:
module.exports = {
isProduction : false,
disableCache: true,
API:"http://alanj.bs.local:15455/",
/////////////////////////////////////
// ENVs
/////////////////////////////////////
MOCKED_BACKEND: 1,
/////////////////////////////////////
}

Answer 2

We ran into the same issue, we have no solution yet but interesting observations after approximately 1 day of trial and error:
Start Packager Script
The 'Start Packager' script in Xcode is responsible for the mess.
It does not pass the environment variables to the packager.
The problem lies within Mac OS open:
Create a new file /tmp/test.command:
#!/bin/bash
echo "Environment Variable XXX='$XXX'"
Now use
XXX="visible" /tmp/test.command
This will print
Environment Variable XXX='visible'
Whereas when called with open:
XXX="not visible" open /tmp/test.command
will just yield:
Environment Variable XXX=''
Even with
XXX="not visible" open --env XXX="abc" /tmp/test.command
we just get:
Environment Variable XXX=''
Not sure if we should file a support ticket at Apple