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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
First of all, MDBT42Q module and breakout board **
ARE NOT 5V COMPATIBLE*** - see all the pink 3.3 boxes next to each pin at https://www.espruino.com/MDBT42Q#pinout - nor are Pixle and Puck.It though may still work when MDBT42Q output is high enough to be detected as high by WS2812B input. Put WS2812B on power and measure voltage over GND and input (IN). If it floats or shows 0V and for sure equal or less than 3.3V (3.6V is absolute max), you are ok to direct connect MDBT42Q output to WS2812B input... ***Make sure you do connect not just INput but also GND,... with separate power supplies this is often missed, and then there is no definitive current flowing in/out of IN (and GND) between MDBT42Q and WS2812B.
If above does not help, you may need to add some switching transistors... see conversation about Logically invert a pin?, where Espruino is driving WSB2812B 'string' to lighten a Nixie Tube Clock. There are multiple options as you will notice.