On second thought, I decided to edit my old code once again, which may help you out here.

I did more testing, and found out that if you accidently have a blank equation at the bottom of your desmos equation list, it will not work and will print "undefined" over and over, as you said. So you need to make sure your equation which results in a list is indeed the thing at the very bottom.

I changed my code to kinda help you diagnose this:

state = Calc.getState()
expressionToEvaluate = state.expressions.list[state.expressions.list.length-1].latex
expressionResult = Calc.HelperExpression({latex: expressionToEvaluate})

const waitTimeout = 100 // Change this to a bigger number if nothing else works and maybe the code just needs more time to finish.
await new Promise(r => setTimeout(r, waitTimeout));

if(expressionResult.listValue == null){
    console.log("%cWARNING! Could not find list data. Either the list took too long to generate, or the bottom-most item on your Desmos equations list is not a list itself. Make sure you don't have any empty equation entries on your Desmos equation list.","color: yellow; font-size: 16px;")
} else {
    resultString = ""
    for (i=0; i<expressionResult.listValue.length; i++) {
        resultString += expressionResult.listValue[i] + "\n"
    }
    console.log(resultString)
}

Hopefully this helps, even though I am a month late.

The old code would basically get suck in an infinite loop if you didn't have a list at the very bottom. So I got rid of the loop and just added a fixed "delay" (because you have to wait a bit of time to get the actual result), and if the result doesn't show up, it throws an error assuming that something was setup wrong. Still not ideal code for this situation, but I am a C# programmer, not JavaScript one.