問題：
Can someone explain to me the difference between NPM , Bower and Composer .有人可以向我解釋NPM 、 Bower和Composer之間的區別嗎？
They are all package managers - correct?他們都是包管理器 - 對嗎？
But when should each one be used?但是什麼時候應該使用每一種呢？
Also, each one appears to have a json file that accompanies it, does this store all the packages you require so they can be installed by cmd line?此外，每個似乎都有一個伴隨它的 json 文件，這是否存儲了您需要的所有軟件包，以便它們可以通過 cmd 行安裝？ Why do you need this file?爲什麼需要這個文件？