In work, I was recently tasked with a data comparison project. A bunch of data was getting duplicated. These records include user first and last names, their phone, email, company.  The master file was a list of these same users but they were assigned "card id" because they were already customers.

The method was to find a match for name (first and last) and if the email didn't match, then separate it out. Then find a match for email, and if the name didn't match, separate it out.

You can imagine this logic:

Go through each line... 
var matches = core.filter(function(a){ return a.firstName == line.firstName && a.lastName == line.lastName; }); // match names 
if (matches.length == 1){ var matchingEmail = matches.filter(function(a){ return a.email == line.email; });
    if (matchingEmail.length == 1){ add it to results }
     else separate it out;
}

And do the same, checking email first. The main feature of Object Oriented Programming is to abstract out data and functionality so that different objects can behave the same way in a system. The main idea of functional programming is that the process can be abstracted out.

What I'm basically doing is, match the line to the map based on some values, if there are any matches, find matches within that result which match some other value. If that result is 1, then we've found a good match, and repeat with the comparison methods swapped.

Or, even more abstractly... running a comparison function, getting results, and running another comparison function to filter the results further.

Let's define our comparison functions:

var compareName = function(a) { return a.firstName == line.firstName && a.lastName == line.lastName; };
var compareEmail = function(a) { return a.email == line.email; };

We can create a method to run these comparisons. It will need the line, the master list, and the two comparison functions.

function compare(line, masterList, comparison1, comparison2)

And then we can call it like so:

compare(line, masterList, compareName, compareEmail);
compare(line, masterList, compareEmail, compareName);

Voila!