TypeScript Never Property Error

Understanding the Error:

This error arises when you attempt to access a property on a value that is of type never. In TypeScript, never is a special type that represents a value that will never occur. It's often used to indicate unreachable code paths or functions that always throw errors.

Common Causes:

1. Unreachable Code:

2. Functions That Always Throw:

3. Incorrect Type Assertions:

Example:

function alwaysThrows(): never {
  throw new Error("This function always throws");
}

const result = alwaysThrows(); // Type of result is never

// This will cause the error:
console.log(result.someProperty); // 'someProperty' does not exist on type 'never'

Solutions:

1. Review Your Code:

   • Carefully examine the code path where the error occurs. Ensure that the code is indeed unreachable or that the function always throws.
   • If the code path is intended to be reachable, check if there's a mistake in your type annotations or logic.

2. Use the never Type Correctly:

   • Use the never type only when you're certain that a value will never occur.
   • Avoid using it unnecessarily, as it can make your code harder to understand.

3. Refactor or Remove Unreachable Code:

4. Handle Errors Gracefully:

Understanding the "'Property does not exist on type 'never'" Error in TypeScript

Scenario 1: Unreachable Code

function alwaysThrows(): never {
  throw new Error("This function always throws");
}

const result = alwaysThrows();
console.log(result.someProperty); // Error: 'someProperty' does not exist on type 'never'

Explanation:

• The alwaysThrows function is explicitly typed to return never, indicating that it will always throw an error and never return a value.
• Since result is assigned the value returned by alwaysThrows, its type is also never.
• Attempting to access the someProperty property on result is invalid because never values are essentially empty; they have no properties or methods.

Scenario 2: Narrowing Types Incorrectly

function checkValue(value: string | number): void {
  if (typeof value === "string") {
    // Type of value is narrowed to string here
    const length = value.length; // No error
  } else {
    // Type of value is narrowed to number here
    const valueSquared = value * value; // No error
  }

  // Outside the if-else block, the type of value is inferred as never
  const unknownProperty = value.someProperty; // Error: 'someProperty' does not exist on type 'never'
}

• Within the if-else block, the type of value is narrowed based on the condition.
• However, outside the block, the type of value is inferred as never because TypeScript cannot determine a common type for both string and number that would guarantee the existence of someProperty.
• Attempting to access someProperty on a never type results in the error.

Scenario 3: Incorrect Type Assertions

const possiblyNever: unknown = null;

const definitelyNever = possiblyNever as never; // Incorrect type assertion

const invalidProperty = definitelyNever.someProperty; // Error: 'someProperty' does not exist on type 'never'

• In this example, a never type assertion is used incorrectly on possiblyNever.
• Even if possiblyNever is null, it doesn't mean it's a never value.
• Asserting it as never leads to the error when trying to access someProperty.

Key Points:

• The never type represents a value that will never occur.
• When you encounter this error, it typically indicates a type mismatch or an unreachable code path.
• Carefully review your code to identify the source of the issue and correct it accordingly.
• Avoid using never type assertions unless you are absolutely certain that the value will never be anything else.

Alternative Approaches to Avoid the "'Property does not exist on type 'never'" Error

Type Guards and Narrowing:

• Explicit Type Guards: Use typeof, instanceof, or custom type guards to narrow the type of a variable based on its value.

  function checkValue(value: string | number): void {
    if (typeof value === "string") {
      // Narrowed type: string
      const length = value.length;
    } else {
      // Narrowed type: number
      const valueSquared = value * value;
    }
  }
  

• Type Assertions: Use type assertions (but with caution) to temporarily assume a specific type for a variable.

  const possiblyString: unknown = "hello";
  const definitelyString = possiblyString as string;
  const length = definitelyString.length; // Assuming it's a string
  

  Note: Overuse of type assertions can lead to runtime errors if the assumption is incorrect.

Optional Chaining and Nullish Coalescing:

• Optional Chaining: Safely access properties of potentially null or undefined values.

  const user: { name?: string; age?: number } = { name: "Alice" };
  const nameLength = user?.name?.length; // If name is null or undefined, nameLength will be undefined
  

• Nullish Coalescing: Provide a default value if a value is null or undefined.

  const username = user?.name ?? "Unknown";
  

Union Types and Discriminated Unions:

• Union Types: Combine multiple types into a single type.

  type Shape = { type: "circle"; radius: number } | { type: "square"; side: number };
  

• Discriminated Unions: Use a common property to distinguish between different types within a union.

  function getArea(shape: Shape): number {
    if (shape.type === "circle") {
      // ...
    } else if (shape.type === "square") {
      // ...
    } else {
      // Handle unexpected types
    }
  }
  

Template Literal Types:

• String Interpolation: Create more expressive type definitions.

  type User = {
    name: string;
    age: number;
  };
  
  type UserKey = keyof User; // Type of keys in User
  
  function getProperty<K extends UserKey>(user: User, key: K): User[K] {
    return user[key];
  }
  

Type Inference and Contextual Typing:

• Let TypeScript Infer Types: Allow TypeScript to infer types based on context.

  const message = "Hello, world!";
  // TypeScript infers the type of message as string
  

• Contextual Typing: Use surrounding context to infer types.

  function greet(name: string): string {
    return `Hello, ${name}!`; // Type of `name` is inferred from the function parameter
  }
  

Custom Type Guards and Utility Types:

• Create Custom Type Guards: Define functions to check for specific conditions.

  function isString(value: unknown): value is string {
    return typeof value === "string";
  }
  

• Utilize Utility Types: Use built-in utility types like Partial, Required, Readonly, etc., to modify existing types.