Saving game data is crucial. Unfortunately, EmulatorJS doesn't save game data naturally; it must be done via Save States. Here's how to save and load your game:
When you want to quit playing, tap on the 3 bars on the upper right corner and select Save State (the floppy disk icon) on the menu. If you want to, you can pause the game when you want to save, though you can save at any point during the game.
Depending on your browser, the download may start automatically or require a confirmation. You'll have to know how to save files downloaded from browsers to your phone.
Wait for the file to finish downloading, then you can leave the emulator page.
Your save state won't automatically load on the next boot for that game, so here's how to load your data the next time you play the game:
Once the game finishes booting up, open the menu via the 3 bars and tap Load State (the opened file icon) on the menu and tap it. Select "Choose File".
Look through your files for the .STATE file with the name of the game you're playing. Tap on the file to load it.
It may take a second, but you'll be placed directly where you left off.
Make sure to Save State every time you want to quit a game, or else you'll lose a lot of progress!
NOTE: It's recommended to delete older .STATE files to save space. Just be sure you're deleting the older files!