Special types in TypeScript – when and how to use any, unknown, never and void in React, Vue, Angular, Express Node.js Apps

This is one of the trickiest but most important parts of TypeScript — the special types: any, unknown, never, and void. These are not primitives, but they give TypeScript extra power for handling real-world scenarios in React, Angular, Vue, and Node/Express apps.

Let’s dive into a clear, structured explanation 👇

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Special Types in TypeScript — any, unknown, never, void

TypeScript provides special types that are not used every day but are essential for handling edge cases in modern apps.

• any → disables type safety (use with caution).
• unknown → safer alternative to any.
• never → represents values that never occur.
• void → represents functions that don’t return anything.

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1. any

What it is

The escape hatch of TypeScript. It tells the compiler:

“Don’t check this. Trust me, I know what I’m doing.”

Any is a friend for beginners in TypeScript because it turns off any type checking, but Any is the an enemy for real commercial projects because it creates weak places in it security and creates possibility to avoid type checking.

In most of real commercial projects using Any in the code of TypeScript is strictly prohibited.

Example

let value: any;

value = "Hello";
value = 42;
value = { key: "value" };

When to use

• Migrating a JavaScript project to TypeScript gradually.
• When working with 3rd-party libraries without type definitions.
• Quick prototyping when exact types are unknown.

Framework Examples

• React: When props come from dynamic data and you haven’t typed them yet.
• Angular/Vue: When working with legacy services with mixed data.
• Express/Node: Accessing req.body without a schema.

// Express example (bad but sometimes necessary)
app.post("/user", (req, res) => {
  const data: any = req.body; // no validation
  console.log(data.randomProperty); // compiler doesn’t complain
});

⚠️ Avoid in production — it removes all type safety.

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2. unknown

What it is

A safer version of any.
You can assign anything to unknown, but you must check its type before using it.

Example

let input: unknown = "Hello";

if (typeof input === "string") {
  console.log(input.toUpperCase()); // ✅ Safe
}

When to use

• When you know a value can be anything, but want to enforce type checking before use.
• Safer handling of user input, API responses, or external libraries.

Framework Examples

• React: Handling data from APIs before mapping into props.
• Angular/Vue: Parsing dynamic JSON responses.
• Express/Node: Validating req.body with runtime checks.

// Express with unknown
app.post("/login", (req, res) => {
  const body: unknown = req.body;

  if (typeof body === "object" && body !== null && "username" in body) {
    const { username } = body as { username: string };
    res.send(`Hello, ${username}`);
  } else {
    res.status(400).send("Invalid request");
  }
});

✅ Encourages type-safe validation.

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3. never

What it is

Represents values that never happen.
A function that throws an error or runs forever has return type never.

Example

function throwError(message: string): never {
  throw new Error(message);
}

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

When to use

• For exhaustive type checks in switch/case statements.
• For error handling functions that never return.

Framework Examples

• React: Ensuring exhaustive props handling.
• Angular/Vue: State management where all cases must be covered.
• Node/Express: Handling impossible conditions in middleware.

// Exhaustive check example (React reducer)
type Action = { type: "ADD" } | { type: "REMOVE" };

function reducer(state: number, action: Action): number {
  switch (action.type) {
    case "ADD": return state + 1;
    case "REMOVE": return state - 1;
    default:
      const _exhaustive: never = action; // compile-time error if new action added
      return state;
  }
}

✅ Helps ensure no forgotten cases.

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4. void

What it is

Represents a function that does not return a value.
Often used for functions that perform side effects.

Example

function logMessage(msg: string): void {
  console.log(msg);
}

When to use

• Event handlers.
• Logging, notifications, DOM updates.
• As a return type when only side effects are expected.

Framework Examples

• React: onClick handlers.
• Angular/Vue: Lifecycle hooks that don’t return values.
• Node/Express: Middleware that only sends a response (no return).

// React
const Button = () => {
  const handleClick = (): void => {
    console.log("Clicked!");
  };

  return <button onClick={handleClick}>Click me</button>;
};

✅ Makes intent clear: “This function does work, but doesn’t return anything.”

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Quick Comparison Table

Type	Meaning	Assignability	Example Use Case
any	Disable type checking	Can assign to anything	Quick prototyping, legacy code
unknown	Type-safe any	Must check before use	API responses, user input
never	Value never occurs	No value can be assigned	Error functions, exhaustive checks
void	No return value	Only undefined or nothing	Event handlers, logging

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Conclusion

• Use any only as a last resort.
• Prefer unknown when type is not known upfront.
• Use never for error functions and exhaustive checks.
• Use void for functions that don’t return anything (like event handlers).

Together, these special types give you more precise control over how TypeScript enforces correctness across React, Angular, Vue, and Node/Express apps.

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