Strict tsconfig | ADR Library

Context

TypeScript's strict flag enables a suite of type-checking options that catch common mistakes: strictNullChecks, strictFunctionTypes, strictBindCallApply, noImplicitAny, noImplicitThis, and more. Without strict: true, TypeScript operates in a lenient mode that allows many unsafe patterns through silently.

Decision

Every project must set "strict": true in its tsconfig.json compilerOptions. Individual strict sub-flags must not be disabled. If a project uses multiple tsconfig files (e.g. for tests), all of them must inherit or set strict: true.

compilerOptions.strict must be true.
Do not set strictNullChecks: false or any other sub-flag to false.

Do's and Don'ts

Do

Set "strict": true in tsconfig.json.
Extend a shared base tsconfig that already has strict enabled.
Fix strict-mode errors rather than disabling the flag.

Don't

Set "strict": false or omit it entirely.
Disable individual strict sub-flags like "strictNullChecks": false.
Use // @ts-ignore to suppress strict-mode errors without justification.

Consequences

Positive

Maximum type safety from the TypeScript compiler.
Catches null/undefined errors, implicit any, and other common pitfalls.
Consistent strictness across all team members and CI.

Negative

Existing projects may need migration effort to fix new errors.
Some third-party type definitions may not be fully strict-compatible.

Enforcement rules

adrs/TS-003-strict-tsconfig.rules.ts

/// <reference path="../rules.d.ts" />

export default {
  rules: {
    "strict-tsconfig": {
      description: 'tsconfig.json must have "strict": true in compilerOptions',
      async check(ctx) {
        const tsconfig = (await ctx.readJSON("tsconfig.json")) as {
          compilerOptions?: { strict?: boolean };
        } | null;

        if (!tsconfig) {
          ctx.report.violation({
            message: "No tsconfig.json found in the project root.",
            file: "tsconfig.json",
            fix: 'Create a tsconfig.json with `"strict": true` in compilerOptions.',
          });
          return;
        }

        if (!tsconfig.compilerOptions) {
          ctx.report.violation({
            message:
              "tsconfig.json is missing compilerOptions — strict mode is not enabled.",
            file: "tsconfig.json",
            fix: 'Add `"compilerOptions": { "strict": true }` to tsconfig.json.',
          });
          return;
        }

        if (tsconfig.compilerOptions.strict !== true) {
          ctx.report.violation({
            message:
              'tsconfig.json does not have "strict": true — strict type checking is disabled.',
            file: "tsconfig.json",
            fix: 'Set `"strict": true` in compilerOptions.',
          });
        }
      },
    },
  },
} satisfies RuleSet;