Flow: process.env type checking bug

[SherloxTV]

Flow version: 0.102.0

Expected behavior

I would like to be able to check process.env variables with if blocks like in #1192.

Actual behavior

The if block has to be placed right before using the process.env variable.

I cannot reproduce in Flow Try cause it does not support process.env. Here is the code:

Working example:

if (!process.env.environment) throw new Error('dbHost missing');

// Get the password secret for the database
const dbSecret = await getDBSecret(process.env.environment)

if (!process.env.dbHost) throw new Error('dbHost missing');
if (!process.env.dbUser) throw new Error('dbUser missing');
if (!process.env.dbName) throw new Error('dbName missing');

// Connect to database and return handle
this.handle = knex({
  client: 'mysql',
  connection: {
    host: process.env.dbHost,
    user: process.env.dbUser,
    password: dbSecret,
    database: process.env.dbName,
  },
})

Breaking example:

if (!process.env.environment) throw new Error('dbHost missing');
if (!process.env.dbHost) throw new Error('dbHost missing');
if (!process.env.dbUser) throw new Error('dbUser missing');
if (!process.env.dbName) throw new Error('dbName missing');

// Get the password secret for the database
const dbSecret = await getDBSecret(process.env.environment)

// Connect to database and return handle
this.handle = knex({
  client: 'mysql',
  connection: {
    host: process.env.dbHost,
    user: process.env.dbUser,
    password: dbSecret,
    database: process.env.dbName,
  },
})

Produces the following error:

Cannot call `knex` with object literal bound to `config` because  null or undefined [1] is incompatible with  string [2] in property `connection.host`.

because Flow do not see that I check the variable anymore if there is another instruction between the checks and the usage.

This is not a blocking bug, it's only preferable for code quality to do all the checks first before actually running the code. And we cannot because of this issue.

Thanks in advance.

[goodmind]

This is limit of refinements, they're invalidated if you're calling functions between them

[SherloxTV]

Why tho ? Calling a function between them will not make it bug .. just makes it cleaner

[goodmind]

You can mutate it between calls

[goodmind]

https://flow.org/en/docs/lang/refinements/#refinement-invalidations-

[SherloxTV]

var obj = { prop: "test" };

function otherMethod() {
  if (Math.random() > 0.5) {
    delete obj.prop;
  }
}

I call this an anti-pattern.

[SherloxTV]

anyway thanks for your answer

[jcready]

You just need to use constants:

const {
  environment,
  dbHost,
  dbUser,
  dbName,
} = process.env;
if (!environment) throw new Error('dbHost missing');
if (!dbHost) throw new Error('dbHost missing');
if (!dbUser) throw new Error('dbUser missing');
if (!dbName) throw new Error('dbName missing');

// Get the password secret for the database
const dbSecret = await getDBSecret(environment)

// Connect to database and return handle
this.handle = knex({
  client: 'mysql',
  connection: {
    host: dbHost,
    user: dbUser,
    password: dbSecret,
    database: dbName,
  },
})

[SherloxTV]

thanks for this great solution @jcready !