TypeScript Strict Mode

Also Known As

TL;DR

Explanation

strict: true in tsconfig.json enables: strictNullChecks (null/undefined are not assignable to other types), noImplicitAny (implicit any is an error), strictFunctionTypes (stricter function parameter checking), strictPropertyInitialization (class properties must be initialised), and more. Most TypeScript bugs caught in the wild are prevented by strictNullChecks alone. Starting with strict: true from day one is far cheaper than enabling it on an existing codebase.

Common Misconception

Why It Matters

Common Mistakes

• Not enabling strict mode on new projects — adding it later requires fixing hundreds of type errors.
• Disabling specific strict checks to silence errors instead of fixing the underlying type issue.
• Using non-null assertion (!) to bypass strictNullChecks — this is a runtime lie; validate instead.
• Not realising strict mode affects third-party library usage — some older libraries have poor types under strict mode.

Code Examples

// Without strictNullChecks — null errors invisible:
function getUsername(user: User): string {
    return user.name.toUpperCase(); // No error even if name is null
}
// Runtime: TypeError: Cannot read properties of null (reading 'toUpperCase')

// tsconfig.json:
{ "compilerOptions": { "strict": true } }

// With strictNullChecks — null must be handled:
function getUsername(user: User): string {
    if (!user.name) return 'Anonymous';
    return user.name.toUpperCase(); // TypeScript knows name is string here
}
// Or with optional chaining:
function getUsername(user: User): string {
    return user.name?.toUpperCase() ?? 'Anonymous';
}

References

• ↗ https://www.typescriptlang.org/tsconfig#strict

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