How 'never' Type Prevents Broken Code in Production

Most production bugs don’t come from syntax errors.
They come from valid code paths that nobody noticed.

TypeScript helps a lot — but there’s one class of bugs it does not catch by default, especially in React codebases that evolve over time.

This article shows:

• a real React bug that silently ships to production
• why TypeScript doesn’t warn you
• how the never type turns that bug into a compile-time error

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What Is the never Type?

In TypeScript, never represents something that cannot happen.

Not “returns nothing”.
Not “empty”.

But this:

A code path that is impossible to reach.

If a value has type never, it means:

• there is no possible runtime value
• if execution reaches here, something is wrong

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Simple Examples

function crash(): never {
  throw new Error("Boom");
}

This function never finishes normally.

function infiniteLoop(): never {
  while (true) {}
}

Execution can never continue past it.

That’s why the return type is never.

---

The One Rule That Makes never Powerful

This rule is the key:

never is assignable to every type,
but no type is assignable to never (except never itself).

Example:

let x: never;

x = "hello"; // ❌ error
x = 123;     // ❌ error

If TypeScript ever sees a real value being assigned to never,
it immediately fails the build.

We’ll use this rule to stop production bugs.

---

The Setup: A Common React Pattern

Imagine a component that renders UI blocks based on a type field
(CMS blocks, feature flags, workflows — very common).

type Block =
  | { type: "hero"; title: string }
  | { type: "cta"; text: string };

Renderer:

function RenderBlock({ block }: { block: Block }) {
  switch (block.type) {
    case "hero":
      return <h1>{block.title}</h1>;

    case "cta":
      return <button>{block.text}</button>;
  }
}

Everything looks fine:

• TypeScript is happy
• CI passes
• No runtime error

---

The Production Bug (Silent)

A teammate adds a new block:

type Block =
  | { type: "hero"; title: string }
  | { type: "cta"; text: string }
  | { type: "banner"; image: string };

They forget to update RenderBlock.

What happens?

• No TypeScript error
• No build failure
• No runtime crash

The component returns undefined.
Nothing renders.

Production UI is broken silently.

This is the worst kind of bug.

---

Why TypeScript Didn’t Save You

From TypeScript’s point of view:

• the function might return JSX.Element | undefined
• that is technically valid
• TypeScript does not assume your switch is exhaustive

So the bug passes.

---

The Wrong “Fix”

You might add a default:

default:
  return null;

This makes things worse:

• still no compiler error
• still broken UI
• now even harder to notice

We need TypeScript to fail loudly.

---

Introducing never

Now we change one line.

function RenderBlock({ block }: { block: Block }) {
  switch (block.type) {
    case "hero":
      return <h1>{block.title}</h1>;

    case "cta":
      return <button>{block.text}</button>;

    default:
      const _exhaustiveCheck: never = block;
      return null;
  }
}

*The result: *

This line is the key:

const _exhaustiveCheck: never = block;

---

What Changed?

This line tells TypeScript:

“If block reaches here, this code is wrong.”

When banner exists, TypeScript now sees:

• block can be { type: "banner" }
• banner is not assignable to never

Compile-time error.
CI fails.
The bug never ships.

---

The Real Win

This is not about exhaustiveness checking.

It’s about this guarantee:

If the code compiles, all cases are handled.

That’s production safety.

---

Why This Matters in Real Codebases

This pattern shines when:

• CMS schemas evolve
• multiple teams touch the same union types
• UI rendering depends on configs or API responses
• reducers or workflows grow over time

Anywhere types change faster than implementations, never protects you.

---

Mental Model

never means:
“This code path must not exist.”

If TypeScript ever proves that it can exist,
your build fails — by design.

---

Conclusion

• TypeScript does not guarantee exhaustive handling by default
• React components can silently break in production
• never turns missing cases into compiler errors
• One line prevents an entire class of bugs

const _exhaustiveCheck: never = value;

Use it where silence is dangerous.