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Push conditionals down

@RiccardoOdone•Suggest a tip•

If you lean on static types and push conditionals down, you may be able to avoid testing glue code with integration tests.

const go = () => {
  const x = doA()
  return is(x) ? doB(x) : doC(x)
}
// ⛔️ Need two integration tests due to the conditional
// ⛔️ If two conditionals are added, need eight tests

const go = () => {
  const x = doA()
  doBOrC(x)
}
// ✅ Need two unit tests in doBOrC()
// ✅ If two conditionals are added,
//    doBOrC() still needs two unit tests.
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