Saving code on Espruino

When you upload code to Espruino normally, it is stored in Espruino's RAM. If you reset the board or power is lost, all your code will be lost.

However it's easy to save your code to flash memory and make it permanent.

Summary

Just type save() on the left-hand side of the IDE and the current state of Espruino including all saved code will be written so that it is loaded at boot time.

However there are other methods of saving code that can be more efficient, or can allow you to do things like saving constants.

You can change methods using the down-arrow below the Upload icon in the IDE, or under Communications in the Settings window. See below for more information.

Boot Process

To understand how best to save data, it's best to know how Espruino loads saved code.

When Espruino starts up, it does a few things:

If Espruino is reset with load() it follows the same steps as above, with hasBeenReset to false. However in Espruino 2v05 and later, load(filename) will follow the same steps but will load the specified file instead of .bootcde/.bootrst.

There are two main ways to save code with Espruino:

save()

If you type save() on the left-hand side of the IDE after your code is uploaded, the contents of Espruino's RAM at that point will be compressed and written in to flash memory.

This includes:

When power is next applied, Espruino will load the information back out of flash and will resume where it left off. You can think of this a bit like 'hibernate' on a PC.

This is the standard way of saving code in normal Espruino devices (it is not suitable for Bangle.js), and it means that you can interact with your code on the left-hand side of the IDE, changing variables and functions, and can then save everything - including your changes.

For instance, if you upload the code var t = E.getTemperature() and type save(), t will contain the temperature of the device at the time that you uploaded - not at the time the device started, or even the time you typed save().

However, this means that any code that was executed at upload time will not be re-executed. For instance you may have some external hardware like an LCD display connected to Espruino - after applying power, the LCD will need to be initialised (since it can not remember its state). In this case you can create a function called onInit (or add a E.on('init', function() { ... }) listener) that is automatically called by the interpreter when it initialises.

Once code is saved, you can return the interpreter to a 'clean' state with reset(). This won't clear out any of the saved data in flash, so if you reboot the device (or call load()) it will re-load your previously saved state. To completely clear out saved code, run reset() and then run save() to save the clean state back into flash memory.

Pros

Cons

Gotchas

Save on Send (to Flash)

Save on Send is an option in the Espruino IDE. Behind the scenes it uses the E.setBootCode command to save JS code directly into Espruino's flash memory. When Espruino boots up, it then executes the JavaScript code.

This is similar to the way you'd program a 'normal' microcontroller.

For instance, if you upload the code var t = E.getTemperature() with Save on Send enabled, t will be set to the temperature every time the device is powered on (in contrast to what happens when you use save().

Save on Send (in the Communications section of the IDE) has three settings:

To remove any code saved with Save on Send, simply call E.setBootCode() with no arguments.

Pros

Cons

Gotchas

To Storage

This is just like Save on Send (to Flash), but on Espruino 2v05 and later you may call load(filename) to reset and load JavaScript code from a named file stored on Espruino.

Combining options

It is possible to combine Save on Send and save() - see Boot Process above for more information.

This allows you to write separate code with Save on Send that can ensure certain things are always done, regardless of the code saved with save().

You can even add files called .boot0, .boot1, .boot2 and .boot3 to add extra code that doesn't interfere with code saved in other ways. You could for instance add the following code:

require("Storage").write(".boot0", `
WIFI_NAME = "MyWiFi";
WIFI_PASS = "HelloWorld123";
`);

To ensure that you always had the variables WIFI_NAME and WIFI_PASS defined regardless of what other code you uploaded.

For instance if you're making a device like the Espruino Home Computer then you might want to use Save on send or .boot0 to save all the code that initialises the display and keyboard. The computer can then be programmed and its state saved with save(), but regardless of what is saved to the device you will always be able to rely on the display and keyboard being set up correctly.

Notes

You may be able to save code to Espruino that puts it into a state that stops you from reprogramming it. On most boards, holding down a button while applying power can be used to force the device to boot without loading any of the saved code. Take a look at the information page on your specific board for more information.

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