Javascript function is executed before it is called

I have a javascript function complaining that a call within its body is not a function before it is called. Could someone please help me explain this?

Context: I am using Meteor and Collection2 and I would like to use a function for reuse at different attributes of my Schemas. Precisely, I would like to do the following:

function foo (autoVal, self){
  if(something){
     return autoVal;
  }else{
     return self.unset();
  }
}

export const myCollection = new Mongo.Collection('myCollection');

const Schema = new SimpleSchema({
   my_field:{
      type:Boolean,
      autoValue: foo(false,this),
   }
});

myCollection.attachSchema(Schema);

When I save this and run Meteor, it does not launch and I get the following error message:

TypeError: self.unset is not a function

I feel that I am missing something about how javascript functions are called or executed, can someone point out why is this happening?

Try this:

function foo (autoVal, self){
  if(something){
     return autoVal;
  }else{
     return self.unset();
  }
}

export const myCollection = new Mongo.Collection('myCollection');

const Schema = new SimpleSchema({
   my_field:{
      type:Boolean,
      autoValue() {
        return foo(false, this);
      }
   }
});

myCollection.attachSchema(Schema);