Why I Enable TypeScript Strict Mode on Day One (And Never Look Back)

The Fear of Strict Mode

Every TypeScript codebase I've joined has had at least one engineer who said "we'll enable strict mode later." Later never comes. I now enable it on line one of every project, and I'm going to show you why the upfront pain is worth it.

What Strict Mode Actually Does

// tsconfig.json
{
  "compilerOptions": {
    "strict": true
  }
}

That single flag enables seven sub-options:

• ▸strictNullChecks — null and undefined are not assignable to every type
• ▸noImplicitAny — every binding must have an explicit type
• ▸strictFunctionTypes — prevents unsafe function type assignments
• ▸strictPropertyInitialization — class properties must be initialized
• ▸noImplicitThis — catches this scoping bugs in functions
• ▸alwaysStrict — emits "use strict" in every file
• ▸strictBindCallApply — type-checks bind, call, apply

The Bug It Caught in Production

On our Ensolite codebase, strict mode flagged this pattern during a PR:

// Without strict: compiled fine, crashed at runtime
function getOrderTotal(order: Order) {
  return order.items.reduce((sum, item) => sum + item.price  item.quantity, 0)

  // ^ item.price could be undefined if the DB row had a NULL column
}

// With strictNullChecks: compile error → fix it before it ships
function getOrderTotal(order: Order) {
  return order.items.reduce((sum, item) => {
    const price = item.price ?? 0  // explicit null handling
    return sum + price  item.quantity
  }, 0)
}

That exact pattern caused a NaN in an order total that slipped through a non-strict codebase at my previous job.

The Real Cost-Benefit

Upfront cost: Maybe 2–4 hours of fixing type errors on a new project, or a multi-day migration on a legacy one.

Ongoing benefit:

• ▸Entire null-deref class of bugs eliminated at compile time
• ▸Prisma and Zod schemas compose cleanly because the types actually match
• ▸Onboarding new engineers is faster — the types are the documentation
• ▸Refactors are safe — the compiler yells if you miss a callsite

If you're starting a new project today, there is no reason not to start with strict mode.