TypeScript Strict Mode

TypeScript Strict Mode

Enable and satisfy strict TypeScript checks including strictNullChecks and exactOptionalPropertyTypes

When to Use

• Starting a new TypeScript project (always enable strict mode from day one)
• Migrating an existing project to stricter type checking
• Debugging type errors caused by strict flags
• Deciding which individual strict flags to enable during incremental adoption

Instructions

1. Enable strict: true in tsconfig.json — this activates all strict flags at once:

{
  "compilerOptions": {
    "strict": true
  }
}

2. What strict: true enables:

   • strictNullChecks — null and undefined are not assignable to other types
   • strictFunctionTypes — function parameter types are checked contravariantly
   • strictBindCallApply — bind, call, apply are fully typed
   • strictPropertyInitialization — class properties must be initialized
   • noImplicitAny — untyped variables/parameters are errors, not any
   • noImplicitThis — this must have a type in function bodies
   • alwaysStrict — emits "use strict" in every file
   • useUnknownInCatchVariables — catch variables are unknown, not any

3. Handle strictNullChecks — the most impactful flag. Check for null before accessing:

function getUser(id: string): User | null {
  /* ... */
}

const user = getUser('123');
// user.name; // Error: Object is possibly 'null'
if (user) {
  user.name; // OK — narrowed to User
}

4. Handle noImplicitAny — add explicit types where inference fails:

// Error: Parameter 'item' implicitly has an 'any' type
function process(item) {
  /* ... */
}

// Fixed
function process(item: unknown) {
  /* ... */
}

5. Handle strictPropertyInitialization — initialize class properties:

class Service {
  // Error: Property 'db' has no initializer
  private db: Database;

  // Fix 1: Initialize in constructor
  constructor(db: Database) {
    this.db = db;
  }

  // Fix 2: Definite assignment assertion (use sparingly)
  private db!: Database;
}

6. Handle useUnknownInCatchVariables:

try {
  await fetchData();
} catch (error) {
  // error is 'unknown', not 'any'
  if (error instanceof Error) {
    console.log(error.message);
  }
}

7. Additional strict flags beyond strict: true:

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "exactOptionalPropertyTypes": true,
    "noPropertyAccessFromIndexSignature": true
  }
}

8. noUncheckedIndexedAccess — index signatures include undefined:

const map: Record<string, string> = { a: 'hello' };
// Without flag: map['b'] is string
// With flag: map['b'] is string | undefined

9. Incremental strict adoption — enable flags one at a time:

{
  "compilerOptions": {
    "strict": false,
    "strictNullChecks": true,
    "noImplicitAny": true
  }
}

Fix errors per flag before enabling the next.

Details

Strict mode catches entire categories of bugs at compile time: null dereferences, implicit any types, uninitialized properties, and unsafe function calls. The upfront cost of fixing strict errors pays back quickly in reduced runtime bugs.

noUncheckedIndexedAccess is not included in strict: true but is one of the most valuable additional flags. It forces you to handle the case where an array index or object key might not exist, preventing undefined is not a function errors.

exactOptionalPropertyTypes distinguishes between "property is missing" and "property is undefined". With this flag, { name?: string } means the property may be absent, but if present it must be a string (not undefined).

Strict migration strategy:

1. Enable strict: true and count the errors
2. If manageable (<100), fix them all at once
3. If large (>100), disable strict and enable flags individually: noImplicitAny first (most errors), then strictNullChecks, then the rest
4. Use // @ts-expect-error temporarily for errors you cannot fix immediately — track them with a lint rule

Trade-offs:

• Strict mode increases type annotation verbosity — but reduces debugging time
• strictNullChecks causes the most migration pain — but prevents the most common runtime error (null dereference)
• noUncheckedIndexedAccess adds | undefined to every index access — use non-null assertion (!) or destructuring with defaults for known-safe accesses
• Third-party libraries with loose types can produce cascading strict errors — use declaration merging to fix them locally

Source

https://typescriptlang.org/tsconfig#strict

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.