TypeScript Any Type: When and How to Use the Escape Hatch

Most developers approach any in TypeScript with guilt. We've been told it defeats the purpose of TypeScript, that it's a crutch for lazy programmers. But this misses the point entirely. The any type isn't a failure of TypeScript's design. It's one of its most thoughtful features.

TypeScript's creators understood a fundamental truth: perfect type safety is less valuable than gradual type safety. They built any as a bridge, not a destination. When converting a 50,000-line JavaScript codebase, demanding perfect types from day one would make migration impossible. The pragmatic approach starts with the use of any and gradually moves toward full type safety over time.

As a result, instead of asking "How do we avoid any?" we should ask "How do we use any effectively on our journey toward better types?"

The Migration Reality

JavaScript projects accumulate complexity over time. That utility function from three years ago handles a dozen different data shapes. The API integration talks to services with evolving schemas. The plugin system accepts arbitrary data from third parties.

When we add TypeScript to this mix, we face a choice. We can spend months creating perfect type definitions for every edge case, or we can strategically use any to make progress. The second approach gets us TypeScript's benefits immediately while leaving room for improvement.

This philosophy inspired tools like Airbnb's ts-migrate, which automatically converts JavaScript codebases to TypeScript by inserting any types everywhere. Rather than trying to infer correct types, it makes the code compile first, then lets developers improve types incrementally. This approach proved so successful that Airbnb migrated over 1.6 million lines of code to TypeScript.

What ts-migrate does automatically, developers can do manually on smaller codebases. The strategy remains the same: add any liberally to make the code compile, then improve types incrementally where it matters most.

// After initial migration: everything compiles but uses any
function processUserData(userData: any): any {
  return {
    id: userData.id,
    name: userData.name,
    email: userData.email,
    lastLogin: userData.lastLogin,
  };
}

// Gradual improvement: add types where impact is highest
interface User {
  id: string;
  name: string;
  email: string;
  lastLogin: Date;
}

function processUserData(userData: any): User {
  return {
    id: userData.id,
    name: userData.name,
    email: userData.email,
    lastLogin: new Date(userData.lastLogin),
  };
}

This pattern shows the ts-migrate philosophy in action. We accept untyped input but guarantee typed output. The function boundary becomes a conversion point where any transforms into something more specific.

The second version accepts any input but immediately converts it to known types. This gives us type safety for everything downstream while acknowledging that the input might come from an untyped source.

Unknown vs Any

Once we understand any as a migration tool, we can make better decisions about when to use its safer cousin, unknown. When we genuinely don't know what shape data will have, unknown forces us to check before using it:

const configData: unknown = JSON.parse(configFile);

if (
  typeof configData === "object" &&
  configData !== null &&
  "apiUrl" in configData
) {
  const apiUrl = configData.apiUrl; // TypeScript knows this exists
}

But unknown is less useful during migration. If we're converting JavaScript code that already works, we often do know what shape the data has. We just haven't formalized that knowledge in types yet. In these cases, any with gradual improvement is more practical than unknown with immediate validation.

The Improvement Path

The most important thing about any is that it should be temporary. Even when it's the right choice initially, we can usually improve the typing over time.

We should start with the data that flows through the most parts of our application:

// Phase 1: Identify critical any usage
interface Article {
  id: string;
  title: string;
  author: any; // Flows through many components
  metadata: any; // Used in fewer places
}

// Phase 2: Type the most impactful data first
interface Author {
  id: string;
  name: string;
  email: string;
}

interface Article {
  id: string;
  title: string;
  author: Author; // Now properly typed
  metadata: any; // Still any, but lower priority
}

This incremental approach means each improvement has maximum impact. We get better type safety where it matters most, without requiring a massive refactoring effort.

TypeScript's compiler options support this gradual approach. We can start with noImplicitAny: false and enable it once we've cleaned up the most important any usages. This prevents new any types from being introduced while giving us time to address existing ones.

Documentation and Intent

When writing new code that requires any, documentation becomes crucial. Unlike legacy migration scenarios where any is a temporary necessity, new code using any should always explain why.

// Using any because third-party analytics library has incorrect type definitions
// TODO: Create custom .d.ts file once library API stabilizes
const analyticsData: any = Analytics.getSessionData();

This comment transforms any from a mystery into a conscious decision. We know why we used it, and we have a plan for improving it.

Type assertions serve a similar purpose. When we know more about data than TypeScript does, assertions document our knowledge:

interface UserPreferences {
  theme: "light" | "dark";
  fontSize: number;
}

const preferences = JSON.parse(
  localStorage.getItem("preferences") || "{}"
) as UserPreferences;

This isn't as safe as proper validation, but it's safer than raw any because it documents what we expect the data to look like. The key insight is that any should never be silent. It should always carry information about why it exists and how it might be improved.

Based on what we've explored about any, in our next article we'll examine modules and namespaces, where proper type organization becomes crucial for larger applications.

The foundation we've built with understanding any will help us make better decisions about when flexibility is valuable versus when strict types provide better guarantees.