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...

var fullName = '';
//Initialize
a variable to store the data

if(data.firstName) //If
there is data, assign it to fullName with a trailing space
    fullName = data.firstName + ' ';
    
if (data.middleInitial)
//If there is data, append it to fullName with a trailing space
    fullName += data.middleInitial + ' ';
    
if (data.lastName)
//If there is data, append it to full name
    fullName += data.lastName;

value = fullName.trim();  //Trim off any trailing spaces and assign the data to the 'value' variable

You can use any Javascript you are comfortable with when author rules like this.  The example above shows the basic sequential construction of a string.  The same thing can be written in a number of ways.  A more developer focused example of this rule is provided below.  Note that this code does the exact same thing as the example above, it is just written in a manner that is less understandable to those unfamiliar with Javascript syntax.  Use whatever is easier or more understable for you.

var fullName = (data.firstName ? data.firstName + ' ' : '') + (data.middleInitial ? data.middleInitial + ' ' : '') + (data.lastName || '');

value = fullName.trim();

The form example described is available for download here.