What does require-dev do in Composer?

Question

Composer

What is the difference between require and require-dev?

Answer 1

Responsible for those dependencies that are needed only for the dev environment.
For example, you know for sure that the phpunit library is needed for development, but it will be superfluous on production, then phpunit gets into require-dev.
When a simple

composer install

dev dependencies are also installed.
When performed

composer install --no-dev

only those dependencies that are in the main require block are installed.
Source