I was developing an Android app using React Native. I was trying to use an emulator to run an application using an Android device, and then trying to run it again as an emulator.
Previously, when using Windows, I solved the problem by using "adb reverse tcp:8081 tcp:8081" method, but this method was not solved in the process of developing with mac today. Please help me somebody.

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Make sure your version of React Native on your Mac is up to date. What do you get when you type react-native -v in your terminal?
Also see: #14246
Getting the same problem on iOS. I haven't tried Android yet.

react-native: 0.53.0
Running npm start -- --reset-cache and then react-native run-ios does not fix the problem.
Frustratingly I have to create an index.ios.js and index.android.js and duplicate the contents of index.js until a fix is identified.
In that case, I'm at a loss.
When you're running react-native run-ios[/android], it's looking for an index.ios[/android].js file, which doesn't exist past react-native 0.51 (or earlier, idk). That's why I thought you were running an older version.
I'm sure this is a recurring issue and somebody else will be able to answer your question, sorry I couldn't be of more help.
No worries @AdrianMrn, all discussion is welcome. It's very bizarre that it has been working in the past but I've only got this problem this morning.
@AdrianMrn
my react-native version is
react-native-cli: 2.0.1
react-native: 0.53.3
@kojongick @hannigand if you still haven't fixed this, a temporary workaround could be to create 2 files called index.android.js and index.ios.js, and copy+pasting what you have in index.js into these files.
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I know this issue is closed, but to anyother else out must know this happened after a react native upgrade. I'm currently using:
RN: 0.56
RN-CLI: 2.0.1
My guess the server is running with a different version and looking for index.[android/ios].js file. Although if I create a new project, It runs as expected.
React Native Cli --> 2.0.1
React Native --> 0.57.2
And I still get the same error. Looks like the only to get out is to create index.android/ios.js file and duplicate the files.
You can also just create index.android.js in the root directory and add only following line to it: import './index.js';.
That way, you won't have to duplicate the content of your index.js.
Most helpful comment
@kojongick @hannigand if you still haven't fixed this, a temporary workaround could be to create 2 files called
index.android.jsandindex.ios.js, and copy+pasting what you have inindex.jsinto these files.