What does strict mode do in TypeScript and why is it helpful?

TypeScript's strict mode is a compiler option that enables a group of strict type-checking flags. Setting "strict": true in tsconfig.json is equivalent to enabling all of these individual flags.

strictNullChecks: null and undefined are only assignable to themselves and any (plus undefined to void). This catches many common bugs where you forget to handle null cases.

noImplicitAny: Raises an error on expressions and declarations with an implied 'any' type. Forces you to be explicit about types.

strictFunctionTypes: Enables stricter checking of function types. Makes function parameter types contravariant instead of bivariant.

strictBindCallApply: Ensures bind, call, and apply methods on functions are invoked with the correct argument types.

strictPropertyInitialization: Ensures class properties are initialized in the constructor. Prevents accessing undefined properties.

noImplicitThis: Raises an error on 'this' expressions with an implied 'any' type.

alwaysStrict: Parses in strict mode and emits "use strict" for each source file.

Using strict mode from the start of a project is recommended. It catches many bugs at compile time and leads to more robust, self-documenting code. Migrating an existing codebase to strict mode can be done incrementally by enabling individual flags.