What does strict mode enable and why does it matter?

The strict flag is a shorthand for several compiler options. The most impactful is strictNullChecks, which stops null and undefined from being silently assignable to every type. noImplicitAny rejects parameters and variables whose type would otherwise be an implicit any. Other flags tighten function assignability and class property initialization.

function greet(name: string | null): string {
  if (name === null) {
    return "Hello, guest";
  }

  return "Hello, " + name;
}

const message = greet(null);

What strict changes

• strictNullChecks forces null and undefined to be handled explicitly
• noImplicitAny rejects untyped parameters that would become any
• strictFunctionTypes checks function parameter types more carefully
• strictPropertyInitialization requires class fields to be initialized
• useUnknownInCatchVariables types catch bindings as unknown

Turning strict on in an existing codebase often produces a large error list. Enabling the flags individually lets a team make progress without blocking delivery. The important thing is to have a plan to reach the full set rather than leaving the codebase permanently half-strict.