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Writing to the logs in an Expo app works just like in the browser: use console.log, console.warn and console.error. Note: we don't currently support console.table outside of remote debugging mode.
Recommended: View logs with Expo tools
When you open an app that is being served from XDE or exp, the app will send logs over to the server and make them conveniently available to you. This means that you don't need to even have your device connected to your computer to see the logs -- in fact, if someone opens the app from the other side of the world you can still see your app's logs from their device.
XDE logging pane
When you open a project in XDE, the log window is split in two. Your app logs show up on the right, and the packager logs show up on the left.
XDE also lets you switch between the logs for any device that has opened the app.
Viewing logs with exp
If you use our command line tool exp, packager logs and app logs will both automatically stream as long as your project is running. To stop your project (and end the logs stream), terminate the process with ctrl+C.
Optional: Manually access device logs
While it's usually not necessary, if you want to see logs for everything happening on your device, even the logs from other apps and the OS itself, you can use one of the following approaches.
View logs for an iOS simulator
Option 1: Use GUI log
In simulator, press ⌘ + /, or go to Debug -> Open System Log -- both of these open a log window that displays all of the logs from your device, including the logs from your Expo app.
Option 2: Open it in terminal
Run instruments -s devices
Find the device / OS version that the simulator you are using, eg: iPhone 6s (9.2) [5083E2F9-29B4-421C-BDB5-893952F2B780]
The part in the brackets at the end is the device code, so you can now do this: tail -f ~/Library/Logs/CoreSimulator/DEVICE_CODE/system.log, eg: tail -f ~/Library/Logs/CoreSimulator/5083E2F9-29B4-421C-BDB5-893952F2B780/system.log