Strict Mode

TL;DR

"strict": true in tsconfig.json is a master switch that simultaneously enables several rigorous type-checking rules. Running TypeScript without strict mode is often considered worse than writing plain JavaScript because it gives a false sense of security.

Mental Model

How It Works

When you enable "strict": true, you are enabling these critical rules:

1. noImplicitAny: If TS cannot figure out the type of a variable or parameter, it defaults to any. This flag throws an error if that happens, forcing you to write an explicit type.
2. strictNullChecks: In non-strict mode, null and undefined can be assigned to any type (like string or number). This flag fixes that “billion-dollar mistake”, ensuring null is its own type and forcing you to check for it.
3. strictPropertyInitialization: Forces you to initialize all class properties in the constructor.
4. strictBindCallApply: Ensures that fn.call() and fn.apply() are used with the correct arguments.

Example

// --- WITHOUT STRICT MODE ---
function logUser(user) { // user is implicitly 'any'
    console.log(user.name);
}
let username: string = null; // Perfectly fine in non-strict mode!


// --- WITH STRICT MODE ENABLED ---
// ERROR: Parameter 'user' implicitly has an 'any' type.
// function logUserStrict(user) { ... } 

// CORRECT:
function logUserStrict(user: { name: string }) {
    console.log(user.name);
}

// ERROR: Type 'null' is not assignable to type 'string'.
// let usernameStrict: string = null; 

// CORRECT:
let usernameStrict: string | null = null;
if (usernameStrict !== null) {
    console.log(usernameStrict.toUpperCase());
}

Common Interview Questions

Should I use strict mode on a new project?

Always. There is zero debate in the TS community about this. Starting a new project without strict mode defeats the entire purpose of using TypeScript.

How do I enable strict mode on an existing, massive legacy JavaScript codebase?

You shouldn’t enable "strict": true immediately, as you will get thousands of errors. Instead:

1. Enable strict in the config.
2. Manually set all the sub-flags (like noImplicitAny and strictNullChecks) to false.
3. Gradually turn the sub-flags to true one by one, fixing the errors across the codebase incrementally.