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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@allObjects
There were such issues a while ago - and you will find posts about it - where correct (and non-poking) code was running and doing that. In some instances it would just increase the byte / character value by some value, such as, "m" became "n", etc. If source code mutilation still happens, (most likely) the upload communication if faulty... and that can be many things.
For me it happened when something was running and making Espruino busy, and the upload did not stop it. Try pressing reset, reconnect, upload again. If this does not help, check your cable... also: what about your power: is something else - in addition to the board - supplied by the connected USB?
I mentioned non-poking code... I guess your code does not - literally or otherwise - poke around in the memory... ;-)