linter-config-typescript

name	linter-config-typescript
description	Fix "Function declarations are not allowed inside blocks in strict mode when targeting ES5" in TypeScript files. Use when linter configs or TS compiler API code fail with this error.

Linter Config and TypeScript Strict Mode

When to Use

• Error: "Function declarations are not allowed inside blocks in strict mode when targeting 'ES5'. Modules are automatically in strict mode."
• Code uses function name() {} inside if, for, callbacks, or other blocks
• Custom lint scripts or configs that use the TypeScript compiler API

Cause

In strict mode with ES5 target, function declarations are only allowed at the top level of a script or the direct body of another function. They are not allowed inside blocks (e.g. if, else, for, while, or callback bodies).

Solution

Replace function declarations with arrow functions or function expressions:

// ❌ Fails
ts.forEachChild(node, (n) => {
  function visit(child: ts.Node) {
    // ...
  }
  visit(n);
});

// ✅ Correct
ts.forEachChild(node, (n) => {
  const visit = (child: ts.Node) => {
    // ...
  };
  visit(n);
});

Checklist

1. Locate function name() {} inside blocks (not at top level)
2. Replace with const name = () => {} or const name = function() {}
3. Ensure the function is still invoked correctly (no change to call sites)
4. Run the script to verify: bun packages/paideia-backend/scripts/lint-project.ts

Reference

• Incident report: release-notes/incidents/2026-03-03-strict-mode-function-declarations-in-blocks.md