How to use variables' value in my java-script in neload?

I just create a variable in Neoload,which name is CenterID.
Its value is from a "csv" file with 5000+ data.
And each time it choose one random value as its current value.

Now I need combine at most 10 values from this file and I plan to call this variable several times and then do the combination job in a java script.

But I found that when I use " var newVar= context.variableManager.getValue("CenterID"); "
null returns as value of newVar.

Could anybody know how to use this "file" type variable in a script?

Or better ways to achieve my goal?