"invalid attempt to spread non-iterable instance" error only for android when trying string spread

Environment

React Native Environment Info: System: OS: macOS High Sierra 10.13.6 CPU: x64 Intel® Core™ i7-4870HQ CPU @ 2.50GHz Memory: 809.96 MB / 16.00 GB Shell: 3.2.57 - /bin/bash Binaries: Node: 8.9.2 - /usr/local/bin/node Yarn: 1.0.2 - /usr/local/bin/yarn npm: 6.3.0 - /usr/local/bin/npm Watchman: 4.9.0 - /usr/local/bin/watchman SDKs: iOS SDK: Platforms: iOS 12.0, macOS 10.14, tvOS 12.0, watchOS 5.0 Android SDK: Build Tools: 26.0.1 IDEs: Android Studio: 3.1 AI-173.4907809 Xcode: 10.0/10A255 - /usr/bin/xcodebuild npmPackages: react: ^16.5.0 => 16.5.0 react-native: ^0.57.2 => 0.57.2 npmGlobalPackages: create-react-native-app: 1.0.0 eslint-plugin-react-native: 3.2.0 react-native-cli: 2.0.1 react-native-gesture-handler: 1.0.0-alpha.29 react-native-git-upgrade: 0.2.7 react-native-rename: 2.2.2 react-native-vector-icons: 4.4.2

Description

In debug mode everything is working as expected. But after bundling, when using spread operation to convert string to array, only Android (platform independent) throws error. But ios works well (both debug and production). The code was working well before updating RN 55.4 to 57. After update I had to change the code to this working example:

Array.from(str).map((p, i) => {return p;})

Reproducible Demo

Error example:

const str = 'error test';
[...str].map((p, i) => {return p;})

The approach is right but what is the problem with babel 7?

[helleroy]

We encountered this as well:

• Works fine in dev-mode on both iOS and Android
• Works fine with release bundle on iOS
• Hard crash with release bundle on Android

After some testing we found that setting loose to true for @babel/plugin-transform-spread solves the problem. This indicates that the problem either lies with Babel 7 or with how react-native/metro handles the bundling of Android apps using Babel 7.

"plugins": [
   ["@babel/plugin-transform-spread", {
     "loose": true
   }]
 ]

Doing this may cause problems in other parts of our code though, so we elected not to use it.

In the end we ended up using Array.from(str) instead of [...str] as mentioned in the description. However, considering how this breaks what is literally the first example in the MDN docs, it should probably be looked into.

[sibelius]

[...Array(3).keys()] is causing trouble on android as well