migrate-to-shoehorn

Migrate to Shoehorn

Why use 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.

The problem with as in tests:

• Trained to avoid using it
• Must manually specify the target type
• Needs double-as for intentionally wrong data (as unknown as Type）

Installation

npm i @total-typescript/shoehorn

Migration patterns

Large objects where you only need a few 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 which

Function	Use case
fromPartial()	Pass partial data that still type-checks
fromAny()	Pass intentionally wrong data (preserving autocomplete)
fromExact()	Enforce a full object (can later switch to fromPartial)

Workflow

1. Gather requirements - Ask the user:

   • Which test files' as assertions are causing problems?
   • Are you dealing with large objects but only care about some properties?
   • Do you need to pass intentionally wrong data to test error paths?

2. Install and migrate：

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