Hide Unsafe Type Assertions in Well-Typed Functions

Overview

Keep type signatures clean; hide assertions in implementations.

If a function needs a type assertion or any internally, that's OK - as long as the public signature is correct. Users see a well-typed API; the unsafe code is contained.

When to Use This Skill

• Function implementations need type assertions
• TypeScript can't follow your logic
• Wrapping libraries with poor types
• Internal complexity, clean external API

The Iron Rule

Never compromise type signatures for implementation convenience.
Hide assertions inside well-typed functions.

Remember:

• Type signatures are your API contract
• Implementations are hidden details
• Users shouldn't see assertions
• Test assertion-heavy code thoroughly

Detection: Exposed Unsafety

// Bad: unsafe return type exposes any
async function fetchPeak(peakId: string): Promise<unknown> {
  return checkedFetchJSON(`/api/peaks/${peakId}`);
}

// Every caller must assert:
const peak = await fetchPeak('denali') as MountainPeak;  // Tedious!

Better: Hide the Assertion

async function fetchPeak(peakId: string): Promise<MountainPeak> {
  return checkedFetchJSON(`/api/peaks/${peakId}`) as Promise<MountainPeak>;
}

// Callers get clean types:
const peak = await fetchPeak('denali');
//    ^? MountainPeak

The assertion is hidden inside; callers get a clean API.

Validate Inside Hidden Assertions

async function fetchPeak(peakId: string): Promise<MountainPeak> {
  const maybePeak = await checkedFetchJSON(`/api/peaks/${peakId}`);
  
  // Validation adds safety to the assertion
  if (!maybePeak || typeof maybePeak !== 'object' || !('name' in maybePeak)) {
    throw new Error(`Invalid peak data: ${JSON.stringify(maybePeak)}`);
  }
  
  return maybePeak as MountainPeak;
}

Now the assertion has runtime backup.

When TypeScript Can't Follow

function shallowEqual(a: object, b: object): boolean {
  for (const [k, aVal] of Object.entries(a)) {
    if (!(k in b) || aVal !== b[k]) {
      //                     ~~~~
      // Element implicitly has an 'any' type
      return false;
    }
  }
  return Object.keys(a).length === Object.keys(b).length;
}

We know k in b is true, but TypeScript doesn't connect this to b[k].

Wrong Fix: Weaken the Signature

// DON'T: weakening b to any exposes unsafety
function shallowEqual(a: object, b: any): boolean {
  // ...
}

shallowEqual({x: 1}, null);  // No error! Crashes at runtime.

Right Fix: Hide the Assertion

function shallowEqual(a: object, b: object): boolean {
  for (const [k, aVal] of Object.entries(a)) {
    // Hidden assertion - we've checked k in b
    if (!(k in b) || aVal !== (b as any)[k]) {
      return false;
    }
  }
  return Object.keys(a).length === Object.keys(b).length;
}

shallowEqual({x: 1}, null);
//                   ~~~~
// Argument of type 'null' is not assignable to parameter of type 'object'.

Type signature stays clean; assertion is narrowly scoped.

Function Overloads as Hidden Assertions

// Overload presents clean signature to callers
async function fetchPeak(peakId: string): Promise<MountainPeak>;
async function fetchPeak(peakId: string): Promise<unknown> {
  return checkedFetchJSON(`/api/peaks/${peakId}`);
}

const denali = fetchPeak('denali');
//    ^? Promise<MountainPeak>

The implementation returns unknown, but callers see MountainPeak.

Narrow Scope of Assertions

// DON'T: Assertion on whole object
const config: Config = {
  a: 1,
  b: 2,
  c: { key: value }
} as any;  // No checking on a, b!

// DO: Assertion only on problem area
const config: Config = {
  a: 1,
  b: 2,  // These are still checked
  c: { key: value as any }  // Only this is unsafe
};

Document Why Assertions Are Valid

function shallowEqual(a: object, b: object): boolean {
  for (const [k, aVal] of Object.entries(a)) {
    // `(b as any)[k]` is safe because we've verified `k in b`
    if (!(k in b) || aVal !== (b as any)[k]) {
      return false;
    }
  }
  return Object.keys(a).length === Object.keys(b).length;
}

Comments help future maintainers understand the assertion.

Test Thoroughly

Functions with hidden assertions need extra testing:

describe('fetchPeak', () => {
  it('handles valid peak data', async () => {
    const peak = await fetchPeak('denali');
    expect(peak.name).toBe('Denali');
    expect(peak.elevationMeters).toBe(6190);
  });

  it('throws on invalid data', async () => {
    mockFetch({ invalid: 'data' });
    await expect(fetchPeak('unknown')).rejects.toThrow('Invalid peak');
  });

  it('handles missing fields', async () => {
    mockFetch({ name: 'Partial' });  // Missing fields
    await expect(fetchPeak('partial')).rejects.toThrow();
  });
});

Pressure Resistance Protocol

1. "Just Change the Return Type"

Pressure: "Make it return unknown to avoid the assertion"

Response: That pushes unsafety to every caller.

Action: Keep clean signature; hide assertion in implementation.

2. "Assertions Are Dangerous"

Pressure: "We should avoid assertions entirely"

Response: Sometimes they're necessary. Contained and tested is OK.

Action: Hide, validate, document, and test.

Red Flags - STOP and Reconsider

• Function signatures containing any or unknown for convenience
• Assertions scattered across calling code
• Changing signatures to avoid implementation errors
• Untested assertion-heavy code

Common Rationalizations (All Invalid)

Quick Reference

// DON'T: Expose unsafety in signature
function fetch(): Promise<unknown> { ... }
const data = await fetch() as Data;  // Assertion at every call site

// DO: Hide assertion in implementation
function fetch(): Promise<Data> {
  return api.fetch() as Promise<Data>;  // Hidden, one place
}

// DO: Narrow scope
const obj = { a: value as any };  // Not: whole object as any

// DO: Document and test
// This assertion is valid because... [explanation]
return data as Data;

The Bottom Line

Hide unsafe code; expose clean types.

Type assertions and any types are sometimes necessary. Keep them in function implementations, not signatures. Document why they're valid. Test thoroughly.

Reference

Based on "Effective TypeScript" by Dan Vanderkam, Item 45: Hide Unsafe Type Assertions in Well-Typed Functions.