Cleanest way to set up a chain of fallbacks?

Question

So, I am geocoding addresses from the Google Places API. There are 38 Google address component types, and we have ~5 in-house address component types, so I need a system of mapping a number of Google types to our in-house types, with fallbacks.

For example:

var neighbourhood = googleResponse.neighborhood || googleResponse.sublocality_level_2 || googleResponse.sublocality_level_1

What is the best/cleanest/most efficient way to do this? Basically, I need to assign a var to some returned value, unless that returned value is undefined, and then move down the chain.

Joe Clay · Accepted Answer · 2016-05-23T10:19:07.247

I think your approach is sound - using the short-circuiting ORs feels a lot cleaner than having some sort of branching statement to me. My two recommendations would be:

You should probably add some kind of hard-coded value (an empty string, perhaps? Depends on your use case) as a final fallback, so that if all of the options aren't found, you don't end up with the variable being set to something unexpected. I think being 100% explicit about all the possible things that could be returned is always a good practice.
I'd recommend pulling that logic out into a function, so that if you want to reuse it elsewhere it'll be more maintainable. Even if you don't think you'll need to use it again, it's still worth doing in case you change your mind later on - having to go through and replace every instance of a piece of code is incredibly tedious! It also makes your code a lot easier to test - you can pass in a hard-coded googleResponse and check that the output matches your expectations.

So my approach would probably look something like this:

function getNeighborhood(googleResponse) {
    return googleResponse.neighborhood
        || googleResponse.sublocality_level_2
        || googleResponse.sublocality_level_1
        || "";
}

var neighborhood = getNeighborhood(googleResponse);

EDIT:

In response to your comment, there's a couple of options I can think of for values that have to be concatenated.

If it's just a matter of "everything or nothing", you could write this quite elegantly with a ternary expression:

function getStreet(googleResponse) {
    return googleResponse.street_number && googleResponse.route
        ? googleResponse.street_number + googleResponse.route
        : "";
}

However, if you start trying to nest these with fallbacks, it's going to get very unreadable very fast. At that point, you're better off just relying on a if/else if/else - it's a bit long winded, but it's comprehensible, and that's more important in my eyes.

function getStreet(googleResponse) {
    if (googleResponse.street_number && googleResponse.route) {
        return googleResponse.street_number + googleResponse.route;
    }
    else if (/* fallback */) {
        return /* concat here */;
    }
    else {
        // Make sure to always return a value, even if all of
        // the other fallbacks fail!
        return "";
    }
}

The only tricky situation is one wherein a value is the concatenation of two strings, eg `var street = googleResponse.street_number + googleResponse.route`... In this case I have to check the existence of two values which is more complicated. — j_d, May 23 '16 at 09:37

Cleanest way to set up a chain of fallbacks?

1 Answers1