design·lab

Core principle

DRY — Don't Repeat Yourself

Every piece of knowledge should have one authoritative source — duplicate it, and you'll eventually update one copy and forget the rest.

✗ The problem

The same rule, copy-pasted three times

// cart.js
const tax = price * 0.08;

// invoice.js
const tax = price * 0.08;

// report.js
const tax = price * 0.08;
cart.js
rate = 0.08
invoice.js
rate = 0.08
report.js
rate = 0.08
The tax rate changes → you must find and edit every copy. Miss one file and you ship a silent pricing bug.
✓ Refactor

Extract a single source of truth

One function owns the rule. Every caller imports it instead of restating it.

// tax.js
export const TAX_RATE = 0.08;

export function taxFor(price) {
  return price * TAX_RATE;
}
taxFor()
single source
↑
cart.js
imports it
invoice.js
imports it
report.js
imports it
✓ See it live

Change the rate — watch which side breaks

Duplicated copies must all be edited by hand. A single source updates every consumer at once, automatically.

✗ Duplicated (3 hand-edited copies)

cart.js
8%
invoice.js
8%
report.js
8%

✓ Single source (one edit)

TAX_RATE
8%
cart.js
8%
invoice.js
8%
report.js
8%
Rate: 8% everywhere — click to change
✓ Takeaway

One source of truth, edited with care

  • Fewer bugs: a rule stated once can't drift out of sync with itself.
  • Easier change: one edit, one place, every caller updates instantly.
  • Caution: don't DRY out code that only looks similar — forcing a shared abstraction onto coincidental duplication costs more than the duplication did.
  • Rule of three: wait until you've copied something a third time before extracting it. Two copies might just be a coincidence.
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