In NeoLoad design section if you drag and drop a Javascript from the action area you have a squeleton of examples like how to get a value from a variable or how to store a value in a new variable with the setValue method.

For instance this is the syntax:

context.variableManager.setValue("computedVar",computedValue); where "computedVar" is the name of the variable that will be created and "computedValue" is here a variable that contains the content that you will store into that variable.