How to remove duplicate code with the DRY principle

DRY — Don't Repeat Yourself — means every piece of knowledge should have a single authoritative representation. When you change a rule, you change it in one place and it updates everywhere. Duplication means changing it in every copy and missing at least one.

// Duplication — same validation in three places
function createUser(email) {
  if (!email.includes('@')) throw new Error('Invalid email');
  ...
}
function updateEmail(email) {
  if (!email.includes('@')) throw new Error('Invalid email');
  ...
}
function inviteUser(email) {
  if (!email.includes('@')) throw new Error('Invalid email');
  ...
}

Extract the duplicated logic once:

function assertValidEmail(email) {
  if (!email.includes('@')) throw new Error('Invalid email');
}

function createUser(email) { assertValidEmail(email); ... }
function updateEmail(email) { assertValidEmail(email); ... }
function inviteUser(email)  { assertValidEmail(email); ... }

Now email validation is defined once. Any change to the rule changes all three callers automatically.

The rule of three: don't extract on the first duplication. Wait until you see the same code three times — then extract. Two occurrences might be coincidence; three is a pattern.

The abstraction trap: DRY can go wrong when two pieces of code look the same but represent different concepts. Forcing them into one function creates a coupling that makes both harder to change. Ask: are these two uses of the same rule, or two different rules that currently happen to look alike?