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The --strict mode

TypeScript allows us to use the --strict compilation flag to enable all strict type checking options. Enabling --strict enables --noImplicitAny, --noImplicitThis, --alwaysStrict, --strictPropertyInitialization, and --strictNullChecks:

The --strictNullChecks compilation flag enables non-nullable types.
The --noImplicitAny flag forces us to explicitly declare the type of a variable when the type inference system is not able to automatically infer the correct type.
The --alwaysStrict flag forces the TypeScript parse to use the strict mode.
The --noImplicitThis flag forces us to explicitly declare the type of the this operator in functions when the type inference system is not able to automatically infer the correct type.
The --strictPropertyInitialization flag forces class properties to be initialized.

We will learn more about the JavaScript use strict and the this operator in Chapter 6, Understanding the Runtime.

Using the --strict compilation flag makes the TypeScript compiler much stricter. Enabling this option in existing large TypeScript projects can lead to the discovery of many errors that may require a significant effort to fix. For this reason, it is recommended to enable the --strict compilation flag in greenfield TypeScript projects and enable the individual flags (--noImplicitAny, --noImplicitThis, --alwaysStrict, and --strictNullChecks) progressively in existing TypeScript projects.

The --noImplicitReturns compilation flag is not one of the flags enabled by the --strict flag. The flag throws an error when not all code paths in function return a value. It is also recommended that you enable this flag in greenfield TypeScript projects or when possible on existing projects.