Avoiding Angle-Bracket Type Assertions

The angle-bracket type assertion syntax (<Type>value) is deprecated in TypeScript, especially in TSX files where it conflicts with JSX syntax.

Why Avoid <Type> Syntax

• JSX conflict: Incompatible with TSX files (React, Preact, etc.)
• Inconsistent syntax: Modern TypeScript uses as Type
• Reduced readability: Looks like generics or JSX
• Tooling issues: Some tools don't handle it well

Modern Alternative: as Type

Basic Replacement

Deprecated:

const value = <string>someValue;
const num = <number>input;

Modern:

const value = someValue as string;
const num = input as number;

With Complex Types

Deprecated:

const user = <User>jsonData;
const config = <AppConfig>settings;

Modern:

const user = jsonData as User;
const config = settings as AppConfig;

Better: Avoid Type Assertions Entirely

Type assertions bypass type checking and should be avoided when possible.

1. Use Type Guards Instead

Bad:

function processValue(value: unknown) {
  const str = value as string;
  return str.toUpperCase();
}

Good:

function processValue(value: unknown) {
  if (typeof value === 'string') {
    return value.toUpperCase();
  }
  throw new Error('Expected string');
}

2. Use Validation Functions

Bad:

const user = apiResponse as User;

Good:

function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    'name' in value &&
    typeof value.id === 'number' &&
    typeof value.name === 'string'
  );
}

const parsed = apiResponse;
if (isUser(parsed)) {
  const user = parsed;
} else {
  throw new Error('Invalid user data');
}

3. Use Validation Libraries

Best:

import { z } from 'zod';

const UserSchema = z.object({
  id: z.number(),
  name: z.string(),
  email: z.string().email(),
});

type User = z.infer<typeof UserSchema>;

const user = UserSchema.parse(apiResponse);

4. Improve Type Inference

Bad:

const element = document.getElementById('myId') as HTMLButtonElement;

Better:

const element = document.getElementById('myId');
if (element instanceof HTMLButtonElement) {
  element.addEventListener('click', handler);
}

Or use querySelector with type inference:

const element = document.querySelector<HTMLButtonElement>('#myId');
if (element) {
  element.addEventListener('click', handler);
}

5. Use Discriminated Unions

Bad:

function handleEvent(event: Event) {
  const mouseEvent = event as MouseEvent;
  console.log(mouseEvent.clientX);
}

Good:

type AppEvent =
  | { type: 'mouse'; clientX: number; clientY: number }
  | { type: 'keyboard'; key: string }
  | { type: 'custom'; data: unknown };

function handleEvent(event: AppEvent) {
  switch (event.type) {
    case 'mouse':
      console.log(event.clientX);
      break;
    case 'keyboard':
      console.log(event.key);
      break;
    case 'custom':
      console.log(event.data);
      break;
  }
}

TSX-Specific Issues

In TSX files, angle-bracket syntax causes syntax errors:

Will cause syntax error in TSX:

const Component = () => {
  const value = <string>getData();
  return <div>{value}</div>;
};

Must use as syntax:

const Component = () => {
  const value = getData() as string;
  return <div>{value}</div>;
};

Even better - validate properly:

const Component = () => {
  const data = getData();
  if (typeof data !== 'string') {
    throw new Error('Expected string');
  }
  return <div>{data}</div>;
};

When Assertions Are Necessary

If you must use assertions (rare cases):

1. Use as const for Literal Types

const config = {
  apiUrl: 'https://api.example.com',
  timeout: 5000,
} as const;

2. Use as Type (Never <Type>)

const value = unknownValue as SomeType;

3. Document Why It's Safe

const element = document.getElementById('app-root') as HTMLDivElement;

Migration Strategy

1. Find all angle-bracket assertions:

   grep -r '<[A-Z][a-zA-Z]*>' --include="*.ts" --include="*.tsx"
   

2. Replace with as syntax:

   • <Type>value → value as Type

3. Review each assertion:

   • Can it be replaced with a type guard?
   • Can validation library be used?
   • Is it truly necessary?

4. Enable linting:

   {
     "rules": {
       "@typescript-eslint/consistent-type-assertions": [
         "error",
         {
           "assertionStyle": "as",
           "objectLiteralTypeAssertions": "never"
         }
       ]
     }
   }
   

ESLint Configuration

Enforce as syntax and discourage assertions:

{
  "rules": {
    "@typescript-eslint/consistent-type-assertions": [
      "error",
      {
        "assertionStyle": "as",
        "objectLiteralTypeAssertions": "never"
      }
    ],
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}

Summary

Never use angle-bracket syntax:

• Use as Type when assertions are unavoidable
• Prefer type guards over assertions
• Use validation libraries for external data
• Validate at runtime for safety
• Document why assertions are necessary
• Enable linting to enforce consistency