Episode 1: How to Read Legacy Code Without Jumping to Conclusions
The fastest way to break a system is to misunderstand it. Nothing gets misunderstood faster than legacy code—especially when wrapped in a ticket that says "just a small change."
From years in QA and engineering, we've learned this truth: most production issues don't come from bad code. They come from misread code.
Let's talk about how to read legacy NetSuite code before you change anything.
Why "Simple Changes" Break Things
You receive a ticket: "Only run this logic when the vendor is local."
You open the script and see:
JavaScript
if (vendorCountry === 'PH') {
processPayment();
}
Easy fix, right? Add another condition. Ship it.
But what you didn't see yet:
- That script also runs during CSV imports
- It's reused by a scheduled script
- Another team depends on its behavior for reporting
The condition you "just added" suddenly:
- Skips logic in batch processes
- Causes mismatched totals
- Breaks downstream integrations
The mistake wasn't the condition. The mistake was changing behavior without understanding who depends on it.
What Legacy Code Really Is
When reading legacy code, that defensive line you see
JavaScript
if
(!value && value !==
0)
return;
Your first instinct as an engineer: "Why is this so weird?"
The better question: "What broke before this was added?"
That line exists because:
- Zero once caused a false negative
- A form behaved differently than a script
- A workflow injected unexpected values
Legacy code isn't random. It's a fossil record of past incidents.
Where to Actually Start Reading
When we open a legacy NetSuite codebase, we don't start with utils folders or common modules.
We start with triggers.
| Script Type | Entry Point to Check |
|---|---|
| User Event | beforeSubmit, afterSubmit |
| Client Script | saveRecord, fieldChanged |
| Map/Reduce | Entry point function |
| Scheduled | main execution function |
Ask these questions first:
- What user action starts this?
- What record state triggers this path?
- What happens if this runs twice?
This mindset came naturally from QA: "How do I reproduce this?"
As engineers, it keeps us safe.
The Conditions Everyone Wants to Remove
You'll often see something like:
JavaScript
if (executionContext !==
'USERINTERFACE') return;
Engineers ask: "Can we remove this? It's limiting."
Don't.
That condition exists because:
- CSV imports caused duplicate records
- Scheduled scripts retriggered logic
- Web services bypassed validations
Removing it might "clean up" the code—but it also removes protection.
Our rule: If a condition looks oddly specific, assume it's guarding something painful.
What You're Really Trying to Learn
When reading legacy code, we're not asking "How can I improve this?"
We're asking:
| Question | Why It Matters |
|---|---|
| What behavior is being protected? | Shows you what breaks if you're wrong |
| Who depends on this behavior? | Reveals hidden integrations and workflows |
| What user actions trigger this? | Helps you understand the full execution path |
If you can't answer these questions, keep reading.
Don't change what you don't yet understand.
The Helper Function Trap
Engineers often jump straight to helper functions:
if (isEdit && oldAmount !== newAmount && !isApproved) {
calculateAmount(record);
}
The bug is almost never in the helper. It's in the conditions that decide when it runs.
When you skip these guards, you miss:
- Approval state logic
- Record state transitions
- One-time protections
That's how regressions sneak in.
Our Hard Rule: Reading Before Refactoring
We don't refactor while reading legacy code.
Even when:
- Naming is bad
- Logic is duplicated
- Structure is ugly
Why? The ugliest code often has the most tests written around it—just not in code format.
Refactoring without understanding behavior is how:
- Edge cases disappear
- Silent failures appear
- Support tickets multiply
Understanding comes first. Cleanup comes later—if at all.
Build Teams That Read Code Correctly
Reading legacy code isn't just about avoiding bugs. It's about building a culture where engineers understand systems before they change them.
The fastest developers aren't always the safest. The best teams are the ones who ask "Why does this exist?" before asking "How can I change this?"
In Episode 2, we’ll talk about the next trap engineers fall into: putting a correct change in the wrong place. Because even when your logic is right, architecture decisions decide whether it’s safe.
Let's talk about how ATSOURCE can help your NetSuite development team build practices that prevent production issues before they start. Our training programs combine QA thinking with engineering expertise, teaching your team to read code the way it was meant to be read.
Contact us
to learn more about our NetSuite development training and team support services.
Meet the Author
Jona has over a decade of experience in SuiteCloud Development on the NetSuite platform. She specializes in implementing advanced solutions and has led teams in creating high-quality software. Jona holds multiple certifications and has been recognized with awards like the Summit Award and Quality Champion Award.
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