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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.
@DrAzzy
You should definitely be powering it off of an external source if you want to drive that many LEDs! If running it on an external power supply, you need to tie the ground of the LED power supply to that of the Espruino.
The fact that it's getting dim at the end (it's expected that as voltage sags, blue and green would go first), though, implies that the voltage drop is not happening at the source (that would dim everything) but rather along the string.
If the issue persists after switching to an external power supply (I have no idea how you're making it work without one now, quite frankly!), your power and ground connections within the array are insufficient for the current involved. At full blast on 5v, WS2812's are spec'ed to pull 60ma each, so 256 of them will pull a whopping 15A! It sounds like you have power and ground running alongside data (so power to the one on the end is going back and forth along the whole array, rather than taking a more direct route) - tying supply power and ground to the power and ground of the string at multiple points along the string should help.