Why Never type and unreachable code in Typescript? - Purpose & Use Cases

The Big Idea

What if your code could tell you when you're writing lines that will never run?

The Scenario

Imagine you write a program and add extra lines of code that never run because the program stops before reaching them. You keep typing and fixing these lines, but they never do anything.

The Problem

This wastes your time and can hide real problems. You might think your code works, but some parts are just dead weight. It's easy to miss bugs or misunderstand what your program really does.

The Solution

The never type in TypeScript helps you catch these unreachable parts early. It tells you when some code can never run, so you can clean it up or fix your logic. This keeps your code clear and error-free.

Before vs After

✗ Before

function example(x: string | number) {
  if (typeof x === 'string') {
    console.log('String');
  } else if (typeof x === 'number') {
    console.log('Number');
  } else {
    console.log('Unknown'); // This is unreachable
  }
}

✓ After

function example(x: string | number) {
  if (typeof x === 'string') {
    console.log('String');
  } else if (typeof x === 'number') {
    console.log('Number');
  } else {
    const _exhaustiveCheck: never = x;
    return _exhaustiveCheck;
  }
}

What It Enables

You can write safer code that clearly shows when all cases are handled and no hidden errors lurk.

Real Life Example

When building a menu with fixed options, never helps ensure you handle every choice. If a new option is added later, TypeScript warns you to update your code, preventing bugs.

Key Takeaways

Unreachable code wastes time and hides bugs.

never type catches unreachable or impossible cases.

This leads to clearer, safer, and easier-to-maintain code.

Practice

1. What does the never type represent in TypeScript?

easy

A. A variable that can hold any type of value.

B. A type that represents optional values.

C. A function that returns null or undefined.

D. A function that never returns normally, like one that always throws an error or loops forever.

Solution

1. Step 1: Understand the never type meaning

   The never type is used for functions or expressions that never complete normally, such as those that throw errors or run infinite loops.

2. Step 2: Compare with other types

   Other options describe common types like any, null, or optional, which are different from never.

3. Final Answer:

   A function that never returns normally, like one that always throws an error or loops forever. -> Option D

4. Quick Check:

   never means no normal return [OK]

Hint: Never means function never finishes normally [OK]

Common Mistakes:

• Confusing never with void or any
• Thinking never means optional or nullable
• Assuming never is a value type

2. Which of the following is the correct syntax for a function that returns never in TypeScript?

easy

A. function fail(): never { throw new Error('Fail'); }

B. function fail(): void { return; }

C. function fail(): never { return null; }

D. function fail(): string { throw new Error('Fail'); }

Solution

1. Step 1: Check function return type and body

   A function returning never must not return normally. Throwing an error fits this.

2. Step 2: Validate each option

   function fail(): never { throw new Error('Fail'); } throws an error, so it never returns normally. function fail(): void { return; } returns void normally. function fail(): never { return null; } tries to return null, which is invalid for never. function fail(): string { throw new Error('Fail'); } returns string but throws, so return type mismatches.

3. Final Answer:

   function fail(): never { throw new Error('Fail'); } -> Option A

4. Quick Check:

   Throw error matches never return [OK]

Hint: Throw error to return never type correctly [OK]

Common Mistakes:

• Returning a value in a never function
• Using void instead of never
• Returning null or undefined in never

3. What will be the output of this TypeScript code?

function check(value: string | number) {
  switch (typeof value) {
    case 'string':
      return 'string';
    case 'number':
      return 'number';
    default:
      const _exhaustiveCheck: never = value;
      return _exhaustiveCheck;
  }
}
console.log(check(42));

medium

A. 'string'

B. 'number'

C. Compilation error due to unreachable code

D. Runtime error

Solution

1. Step 1: Analyze the switch cases

   The function handles 'string' and 'number' types explicitly, so the default case is unreachable for these inputs.

2. Step 2: Check the call and output

   Calling check(42) matches the 'number' case, so it returns 'number'. The default case with never is never reached.

3. Final Answer:

   'number' -> Option B

4. Quick Check:

   Handled all cases, returns 'number' [OK]

Hint: Check switch covers all types, default never runs [OK]

Common Mistakes:

• Thinking default case runs for known types
• Confusing never with runtime errors
• Expecting compilation error for unreachable default

4. Identify the error in this TypeScript code snippet:

function assertNever(x: never): never {
  throw new Error('Unexpected value: ' + x);
}

function process(value: string | number) {
  if (typeof value === 'string') {
    return 'string';
  } else if (typeof value === 'number') {
    return 'number';
  } else {
    return assertNever(value);
  }
}

medium

A. Error because assertNever should return void not never.

B. Error because assertNever is called with a non-never type.

C. No error; code is correct and safe.

D. Error because value can never be string | number.

Solution

1. Step 1: Understand the function purpose

   assertNever is used to catch unexpected values at compile time by requiring a never type.

2. Step 2: Analyze the process function

   All possible types string and number are handled. The else branch is unreachable, so value is never there, making the call valid.

3. Final Answer:

   No error; code is correct and safe. -> Option C

4. Quick Check:

   Unreachable else means value is never [OK]

Hint: Else unreachable means value is never type [OK]

Common Mistakes:

• Thinking assertNever is called with string or number
• Confusing never return with void
• Assuming else branch always runs

5. Given this TypeScript code, what is the best way to ensure all cases are handled and catch missing cases at compile time?

type Shape = { kind: 'circle', radius: number } | { kind: 'square', size: number };

function area(shape: Shape): number {
  switch (shape.kind) {
    case 'circle':
      return Math.PI * shape.radius ** 2;
    // Missing case for 'square'
  }
}

hard

A. Add a default case that calls a function accepting never to catch unhandled cases.

B. Ignore missing cases; TypeScript will warn automatically.

C. Add a default case that returns 0 without checks.

D. Use any type for shape to avoid errors.

Solution

1. Step 1: Identify missing case problem

   The 'square' case is missing, so the switch is not exhaustive.

2. Step 2: Use never type to catch missing cases

   Adding a default case that calls a function requiring never forces TypeScript to check exhaustiveness and report missing cases.

3. Final Answer:

   Add a default case that calls a function accepting never to catch unhandled cases. -> Option A

4. Quick Check:

   Use never in default to enforce exhaustiveness [OK]

Hint: Use default with never to catch missing cases [OK]

Common Mistakes:

• Assuming TypeScript warns without default never
• Returning default values silently
• Using any to bypass type safety