Support flow experimental.const_params=true

Is this a bug report?

Yes

Have you read the Contributing Guidelines?

Yes

Environment

Environment: OS: macOS High Sierra 10.13 Node: 6.11.2 Yarn: 1.1.0 npm: 3.10.10 Watchman: 4.7.0 Xcode: Xcode 9.0 Build version 9A235 Android Studio: Not Found

Packages: (wanted => installed) react: 16.0.0-alpha.12 => 16.0.0-alpha.12 react-native: ^0.48.4 => 0.48.4

Steps to Reproduce

(Write your steps here:)

1. Create a new react-native app with create-react-native-app
2. yarn add --dev flow-bin@^0.49.1 (same version specified in .flowconfig)
3. Add experimental.const_params=true under [options] in .flowconfig
4. ./node_modules/.bin/flow

Expected Behavior

No errors.

(Write what you thought would happen.)

Actual Behavior

$ flow
node_modules/fbjs/lib/UnicodeBidi.js.flow:90
 90:   fallback = fallback || UnicodeBidiDirection.NEUTRAL;
       ^^^^^^^^ fallback. const param cannot be reassigned (see experimental.const_params=true in .flowconfig)
 89: function resolveBlockDir(str: string, fallback: ?BidiDirection): BidiDirection {
                                           ^^^^^^^^^^^^^^^^^^^^^^^^ const param fallback

node_modules/fbjs/lib/UnicodeBidi.js.flow:92
 92:     return fallback;
                ^^^^^^^^ null. This type is incompatible with the expected return type of
 89: function resolveBlockDir(str: string, fallback: ?BidiDirection): BidiDirection {
                                                                      ^^^^^^^^^^^^^ string enum

…[removed for brevity]…

... 88 more errors (only 50 out of 138 errors displayed)
To see all errors, re-run Flow with --show-all-errors

Reproducible Demo

https://github.com/ckknight/ReactNative-const_params-repro

[ckknight]

This also affects react-native@0.49.0

[kelset]

👋 there - this issue is still technically “not fixed”, for the reason Eli explained above.

Basically every instance similar to this in the main codebase

function foo(arg) {
  arg += 2;
  console.log(arg);
}

would need to be changed to

function foo(arg) {
  let localArg = arg;
  localArg += 2;
  console.log(localArg);
}

There is a codemod Eli worked on -> https://github.com/cpojer/js-codemod/pull/85 which would help a lot in automating this process, but there is a 1% cases in which it would conflict.

So, all in all, there is a lot of work related to “code hygiene” - we’d love to see some PRs to help with this effort but it’s really low prior right now. So I’m going to close it as there are more important tasks to focus on.