Enable TypeScript Strict Mode

Why this matters

TypeScript's default configuration is intentionally lenient to make adoption easier. But the lenient defaults allow categories of bugs that strict mode catches at compile time:

• strictNullChecks: false — allows null and undefined to be assigned to any type. You can call .toString() on a value that is null at runtime and TypeScript won't warn you.
• noImplicitAny: false — function parameters without type annotations silently become any, disabling all type checking.
• strictFunctionTypes: false — allows contravariant function assignment, enabling callbacks with wrong parameter types.

Enabling strict: true is a single flag that turns on all strict checks. It is the difference between TypeScript being a documentation tool and TypeScript being a safety net.

The rule

Set "strict": true in your tsconfig.json compilerOptions. This enables: strictNullChecks, noImplicitAny, strictFunctionTypes, strictBindCallApply, strictPropertyInitialization, noImplicitThis, alwaysStrict, and useUnknownInCatchVariables.

Bad example

// tsconfig.json — NOT strict
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "strict": false
  }
}

// These compile without errors — but crash at runtime
function greet(name) {
  // name is implicitly any — no type checking
  return name.toUpperCase(); // crashes if name is null
}
 
const user = getUser(); // could return null
console.log(user.email); // null access — no warning

Good example

// tsconfig.json — strict mode enabled
{
  "compilerOptions": {
    "target": "ES2022",
    "module": "ESNext",
    "strict": true
  }
}

// TypeScript now catches these at compile time
function greet(name: string) {
  return name.toUpperCase(); // safe — name is guaranteed to be a string
}
 
const user = getUser(); // returns User | null
if (user) {
  console.log(user.email); // safe — null check narrows the type
}

How to detect

Check your tsconfig.json:

grep -n "strict" tsconfig.json

If "strict" is missing or set to false, the project is running without strict mode.

Remediation

1. Set "strict": true in tsconfig.json
2. Run npx tsc --noEmit to see all new errors
3. Fix errors in priority order: null checks first, then implicit any, then function types
4. For large codebases, enable individual flags one at a time: start with strictNullChecks, then noImplicitAny
5. Use // @ts-expect-error sparingly for temporary suppressions with a linked ticket to fix later

References

• TypeScript Handbook: Strict Mode
• TypeScript Strict Mode Explained
• Total TypeScript: Strict Mode