Code splitting: a practical guide

It is rarely the thing that gets a project approved, and often the thing that decides how it goes. This guide covers what code splitting actually involves, where it usually goes wrong, and how to tell whether yours is in reasonable shape.

Speed is a feature people notice only in its absence, and then they leave rather than complain. Nothing below assumes a large team or a large budget — most of it is a decision somebody has to make and then write down.

The reason this keeps coming up

Ship the code for this page, not for every page. The reasoning matters more than the rule, because the rule has exceptions. The failure mode is not doing it wrong, it is doing it once and assuming it stays done.

For most businesses the question is not whether this matters but how much of it is worth doing right now. That depends on what you are trying to achieve in the next few months, not on best practice in the abstract. Getting it slightly wrong is survivable. Ignoring it entirely is not.

The practical version

Route-level splitting is the easiest win. Getting it slightly wrong is survivable. Ignoring it entirely is not. Most teams find the first pass takes an afternoon and the maintenance takes minutes a month.

Performance work is mostly subtraction, which makes it unpopular and effective. The version that works in practice is usually less elaborate than the version described in the guides.

Measure first, because splitting adds requests. The cost of getting this wrong is rarely visible on the day it happens. Doing this properly once is usually cheaper than doing it approximately three times.

A working checklist

If you want a quick read on where you stand, work through this. Anything you cannot answer confidently is where to start.

• Ship the code for this page, not for every page
• Route-level splitting is the easiest win
• Measure first, because splitting adds requests
• Someone is named as the owner, not just assumed to be
• There is a date in the calendar to review it again
• The decision and the reasoning behind it are written down somewhere findable
• You could explain the current setup to a new hire in five minutes

Warning signs

The most common failure is not doing this badly. It is doing it once, during a launch, and never revisiting it. Circumstances move, the setup does not, and the gap widens quietly until something breaks or somebody notices the numbers.

• It was configured during a launch and has not been touched since
• Different people in the business believe different things are true about it
• There is no way to tell whether the last change helped or hurt
• The only person who understands it has left, or is about to

Real-world numbers from actual visitors matter more than a score produced on a fast laptop. Small and consistent beats large and occasional here.

How we approach it

On our projects this gets handled during the build rather than added afterwards, because retrofitting it costs several times more than including it. We write down what was decided and why, so the next person to touch it is not guessing.

If you are working with someone else, the questions worth asking are simple: who owns this, how will we know it is working, and what happens when it needs to change?

Turning this into a decision

Pick the single item from the checklist above that would cause the most trouble if it turned out to be wrong. Fix that one, confirm it worked, then move on. If any of that sounds like a description of your current setup, it is fixable.