typescript-safety

TypeScript safety

Getting the compiler to catch bugs before they run, on TypeScript 6 and 7. TS 7.0 is the Go compiler, installed as plain typescript since July 2026. strict has been the default since 6.0, so the remaining gains come from the flags strict leaves off, from narrowing instead of casting, and from parsing data at the edges.

Types describe code, not data. Anything that enters the process (a response body, JSON.parse, storage, env vars) is unchecked until code checks it. An annotation on an any is a cast.

Every escape hatch turns off the check that would have caught the bug. No any in new code. Reach for unknown and narrow. A cast needs a runtime check next to it.

Source checks and dates: SOURCES.md.

Procedure

1. Read the project's tsconfig.json and the installed typescript version (npx tsc --version). On 5.x or 6.x with deprecation errors, start with upgrade-6-to-7.md.
2. Compare the config with the baseline in config.md. Turning on a flag in an old codebase can add hundreds of errors: count them with tsc --noEmit first, and fix by module.
3. Read the resource for the task in the table below.
4. Write the change. Prefer the first tool that works: narrowing, then a predicate, then a checked cast.
5. Run tsc --noEmit, and eslint if the project has typed linting. Report both results, and say which TS version ran.

data from outside ──> unknown ──> parse at the edge ──> precise types inside
                                  domain-types.md      narrowing.md, generics.md
                                                              |
  tsconfig flags catch the rest: config.md        types from types: type-level.md
                                                              |
  any / as / ! found? escape-hatches.md      lint what tsc cannot: linting.md

Where to read