is-kit Reached 50 Stars ⭐ Here’s How We Use It in Production

Description and use

is-kit is a library for building reusable type guard predicates in TypeScript. In a large application it replaced many duplicated guard functions, cutting code by 249 lines, removing redundant casts, and centralizing primitive semantics. All checks are now composed from small, reusable predicates, and repository rules enforce using them instead of manual conditions.

Problem

Repeated, boilerplate type guard checks for unknown values were scattered across the codebase, leading to duplicated logic, heavy assertion casting, and difficulty extending new checks.

Outcome

Consolidated guard logic reduced code duplication by 249 lines, eliminated repetitive assertion casts, centralized primitive semantics, and established a reusable guard pattern that is now a repository rule.

Workflow

• Install is-kit via pnpm add is-kit
• Create application‑owned guard wrappers that re‑export is-kit primitives
• Replace inline guard logic with composed guards using define, equalsKey, or, arrayOf, oneOfValues, etc.
• Update existing error guard modules to use the new composable pattern
• Add a contributor rule to prefer is-kit combinators over native operators
• Run tests to ensure behavior is unchanged
• Deploy updated code to production

Evidence

The first adoption refactor was not just 👇

pnpm add is-kit

It changed the structure of the guard layer.

| Observable result | Change |
| ----------------- | ------ |
| Error guards | 7 separate modules became 1 shared module |

Source

After using it in the application, the advantages became clearer.

1. Incremental adoption
2. Less assertion casting
3. Shared runtime semantics
4. Normal TypeScript control flow
5. Small dependency surface

Source