Allow failing the build when environment variables are missing

[dnagir]

🙋 feature request

Given the environment variables, I am using are missing in a .env* file
When I parcel build
Then the build should fail and suggest that the env var is missing

This would make it much safer to build the application as it would fail at the build-time rather than when it has already been deployed.

🤔 Expected Behavior

When the JS code refers to the env variables using process.env.FOO, it should fail when such a variable is not set.

😯 Current Behavior

When the JS code refers to the env variables using process.env.FOO, it gets replaced with void 0 (undefined) and succeeds the build.

💁 Possible Solution

• always fail when the environment variable is missing
• need to consider if backwards compatibility is important
• potentially allow to opt-in/out of this behaviour

🔦 Context

I built and deployed the application without realising that the env vars were missing.

[tannerntannern]

I could really use this feature as well. Any plans to add this?