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In this Node.js tutorial on Docker: https://nodejs.org/en/docs/guides/nodejs-docker-webapp/

What is the point of COPY package*.json ./ ?

Isn't everything copied over with COPY . . ?

The Dockerfile in question:

FROM node:8
# Create app directory
WORKDIR /usr/src/app
# Install app dependencies
# A wildcard is used to ensure both package.json AND package-lock.json are copied
# where available (npm@5+)
COPY package*.json ./
RUN npm install
# If you are building your code for production
# RUN npm install --only=production
# Bundle app source
COPY . .
EXPOSE 8080
CMD [ "npm", "start" ]
                I pretty sure that remove COPY package*.json ./ still work if you put RUN npm install under COPY . .  .Same question with you, I think that unnecessary...
– Truong Dang
                Jul 26, 2018 at 8:53
                Does COPY package*.json as well includethe  packge-lock.json file? If not what is the star for then?
– Eldar Omerovic
                Nov 17, 2021 at 12:16
This is a common pattern in Dockerfiles (in all languages).  The npm install step takes a long time, but you only need to run it when the package dependencies change.  So it's typical to see one step that just installs dependencies, and a second step that adds the actual application, because it makes rebuilding the container go faster.
You're right that this is essentially identical if you're building the image once; you get the same filesystem contents out at the end.
Say this happens while you're working on the package, though.  You've changed some src/*.js file, but haven't changed the package.json.  You run npm test and it looks good.  Now you re-run docker build.  Docker notices that the package*.json files haven't changed, so it uses the same image layer it built the first time without re-running anything, and it also skips the npm install step (because it assumes running the same command on the same input filesystem produces the same output filesystem).  So this makes the second build run faster.
                I thought it's a kind of corner cut where, provided npm installfails, we avoid proceeding to copying over application source.
– Jakub Barczyk
                Jul 26, 2018 at 19:58
                But isn't the node_modules directory created in that first layer fully covered by the node_modules directory in the next layer that is copied over with the COPY . . command? Also, there could theoretically be newer module versions in the host dir with unchanged package.json.
– Passiday
                Apr 25, 2020 at 9:24
                You also should include node_modules in a .dockerignore file so that the COPY step doesn't overwrite what just got npm installed.
– David Maze
                Apr 25, 2020 at 11:15
                this is just a lame docker design, please vote my comment and get noticed by docker @docker
– Kevin Simple
                Mar 2, 2022 at 3:01
                I think in terms of consistency, it is good because docker uses the cached node_module. However, i think it won't facilitate the use of minor version in package.json, as it doesn't do npm install again therefore minor version is not downloaded.
– v.ng
                Sep 28, 2022 at 4:06
During building of an image docker works on the basis of layer based architecture that is each line you write in a Dockerfile gets into the layer and gets cached... now the purpose of copying the package*.json file first is a kind of optimization you did in the Dockerfile during the building of an image if bcoz we want to run the command npm install only when some dependencies gets added into the project hence copying first package*.json into the image file system for every successive build runs npm install only when a new dependency gets added into the project and then just copy everything into the image filesystem then after docker is a headless pc of software it doesn't check a layer subsequent to the change of a layer it just executes after then ... hence we get saved each time without running npm install after copying the entire host file system into image file system
        
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