Issues with very basic Field Script Output

(I am moving this from another Forum as the question is more Script based.)

My knowledge of JavaScript is VERY rudimentary although I am generally familiar with object oriented programming and can pick things up quickly with working examples.

I first tried this without being in a function, and didn’t get an error, but then also do not get a result in the Script Field.

SO I changed the script to below and now get an odd error like it can no longer find the very same field record.

(If I “Paste” in from the snippet editor, I get the very same field id so that is not the error!?)

function calculation_result () {
var length = record.getFieldValue('fld-0f39aaf844314fde8b5af7adb5d3bd5f');
var value_unit = record.getFieldValue('fld-616fa42329f347148fa7813db6a3289b');
var result_unit = record.getFieldValue('fld-3148ad66cbb14d5b875ee8b26429cdec');

var result = "";

if (result_unit == "inches") {
   
if (value_unit == "feet") {
	result = length * 12;
} else if (value_unit == "yard") {
   result = length * 36;
} else if (value_unit == "cm") {
	result = length * 2.54;
} else if (value_unit == "metre") {
	result = length * 0.0254;
} else {
   result = length;
} }

if (result_unit == "feet") {
   
if (value_unit == "inches") {
	result = length / 12;
} else if (value_unit == "yard") {
   result = length / 3;
} else if (value_unit == "cm") {
	result = length * 30.48;
} else if (value_unit == "metre") {
	result = length * 0.3048;
} else {
   result = length;
}
}

if (result_unit == "yard") {
   
if (value_unit == "inches") {
	result = length / 36;
} else if (value_unit == "feet") {
   result = length / 3;
} else if (value_unit == "cm") {
	result = length * 0.0109361;
} else if (value_unit == "metre") {
	result = length * 1.09361;
} else {
   result = length;
}
}

if (result_unit == "cm") {
   
if (value_unit == "inches") {
	result = length * 2.54;
} else if (value_unit == "feet") {
   result = length * 30.48;
} else if (value_unit == "yard") {
	result = length * 91.44;
} else if (value_unit == "metre") {
	result = length * 100;
} else {
   result = length;
}
}

if (result_unit == "metre") {
   
if (value_unit == "inches") {
	result = length / 39.37;
} else if (value_unit == "feet") {
   result = length * 0.3048;
} else if (value_unit == "cm") {
	result = length / 100;
} else if (value_unit == "yard") {
	result = length * 0.9144;
} else {
   result = length;
}
}

return result;

}

calculation_result();

This gives me the error:

ReferenceError: Can’t find variable: record. Column:20, line:2

I must be missing something very simple about field declarations that I can’t see.