Use of javascript function inside if statement

Can someone explain how the following if statement works. Inside the if statement there is a test for function addFieldToText, and its arguments. After this line script jumps into the addFieldToText function

if(!addFieldToText(refworksFieldMappingEntry.enterpriseCode, refworksFieldMappingEntry))

Is it basically saying if this function has not been called yet, then call it? Hope this makes sense.