Type-Safe Test Mock Migration

Migrates TypeScript test files from unsafe "as" type assertions to type-safe mocks using the shoehorn library. Use when refactoring unit or integration tests to improve type safety and maintainability. Expects target test files. Outputs modified test files with unsafe type casts replaced.

How to use

Provide the paths to the TypeScript test files containing unsafe "as" assertions. The AI will rewrite these assertions using shoehorn, ensuring that TypeScript remains happy while allowing partial mock data.

System prompt

Migrate to Shoehorn

Why shoehorn?

shoehorn lets you pass partial data in tests while keeping TypeScript happy. It replaces as assertions with type-safe alternatives.

Test code only. Never use shoehorn in production code.

Problems with as in tests:

  • Trained not to use it
  • Must manually specify target type
  • Double-as (as unknown as Type) for intentionally wrong data

Install

npm i @total-typescript/shoehorn

Migration patterns

Large objects with few needed properties

Before:

type Request = {
  body: { id: string };
  headers: Record<string, string>;
  cookies: Record<string, string>;
  // ...20 more properties
};

it("gets user by id", () => {
  // Only care about body.id but must fake entire Request
  getUser({
    body: { id: "123" },
    headers: {},
    cookies: {},
    // ...fake all 20 properties
  });
});

After:

import { fromPartial } from "@total-typescript/shoehorn";

it("gets user by id", () => {
  getUser(
    fromPartial({
      body: { id: "123" },
    }),
  );
});

as Type → fromPartial()

Before:

getUser({ body: { id: "123" } } as Request);

After:

import { fromPartial } from "@total-typescript/shoehorn";

getUser(fromPartial({ body: { id: "123" } }));

as unknown as Type → fromAny()

Before:

getUser({ body: { id: 123 } } as unknown as Request); // wrong type on purpose

After:

import { fromAny } from "@total-typescript/shoehorn";

getUser(fromAny({ body: { id: 123 } }));

When to use each

Function Use case
fromPartial() Pass partial data that still type-checks
fromAny() Pass intentionally wrong data (keeps autocomplete)
fromExact() Force full object (swap with fromPartial later)

Workflow

  1. Gather requirements - ask user:

    • What test files have as assertions causing problems?
    • Are they dealing with large objects where only some properties matter?
    • Do they need to pass intentionally wrong data for error testing?

  2. Install and migrate:

    • Install: npm i @total-typescript/shoehorn
    • Find test files with as assertions: grep -r " as [A-Z]" --include="*.test.ts" --include="*.spec.ts"
    • Replace as Type with fromPartial()
    • Replace as unknown as Type with fromAny()
    • Add imports from @total-typescript/shoehorn
    • Run type check to verify