Strict Mode

Definition

A configuration flag (`"strict": true`) in `tsconfig.json` that simultaneously enables a suite of rigorous type-checking rules, providing the strongest guarantees of program correctness.

Explain Like I'm New

Turning strict mode ON is like playing a video game on 'Hardcore' difficulty. The compiler will catch significantly more bugs, force you to handle every possible `null` value, and refuse to let you use `any` implicitly. It is highly recommended for all new projects.

Real World Example

If `strictNullChecks` (part of Strict mode) is off, TS allows `const x: number = null`. This causes a crash when you do `x.toFixed()`. If Strict mode is ON, TS blocks `null` from being assigned to `number`.

Common Use Cases

•Enterprise app development
•Eliminating 'Cannot read properties of undefined' errors

Interactive Example

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Interview Questions

basic

Should you use `strict: true` for new projects?

intermediate

What does the `noImplicitAny` rule do?

advanced

What is `strictFunctionTypes`?

Flash Cards

Question

What does noImplicitAny do?

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Answer

If you write `function add(a, b)`, TypeScript doesn't know the types, so it secretly infers them as `any`. With `noImplicitAny: true`, TS throws a compile error, forcing you to explicitly type them `(a: number, b: number)`.

Question

What is strictFunctionTypes?

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Answer

It ensures that function parameters are checked contravariantly. This prevents you from assigning a function that takes a broad type (like `Animal`) to a variable that expects a specific type (like `Dog`), which could cause runtime crashes if the function tries to access `Dog`-specific properties.