Why does "npm install" rewrite package-lock.json?

I just recently upgraded to npm@5. I now have a package-lock.json file with everything from package.json. I would expect that, when I run npm install that the dependency versions would be pulled from the lock file to determine what should be installed in my node_modules directory. What's strange is that it actually ends up modifying and rewriting my package-lock.json file.

For example, the lock file had typescript specified to be at version 2.1.6. Then, after the npm install command, the version was changed to 2.4.1. That seems to defeat the whole purpose of a lock file.

What am I missing? How do I get npm to actually respect my lock file?

Update 3: As other answers point out as well, the npm ci command got introduced in npm 5.7.0 as additional way to achieve fast and reproducible builds in the CI context. See the documentation and npm blog for further information.

Update 2: The issue to update and clarify the documentation is GitHub issue #18103.

Update 1: The behaviour that was described below got fixed in npm 5.4.2: the currently intended behaviour is outlined in GitHub issue #17979.

Original answer: The behaviour of package-lock.json was changed in npm 5.1.0 as discussed in issue #16866. The behaviour that you observe is apparently intended by npm as of version 5.1.0.

That means that package.json can override package-lock.json whenever a newer version is found for a dependency in package.json. If you want to pin your dependencies effectively, you now must specify the versions without a prefix, e.g., you need to write them as 1.2.0 instead of ~1.2.0 or ^1.2.0. Then the combination of package.json and package-lock.json will yield reproducible builds. To be clear: package-lock.json alone no longer locks the root level dependencies!

Whether this design decision was good or not is arguable, there is an ongoing discussion resulting from this confusion on GitHub in issue #17979. (In my eyes it is a questionable decision; at least the name lock doesn't hold true any longer.)

One more side note: there is also a restriction for registries that don’t support immutable packages, such as when you pull packages directly from GitHub instead of npmjs.org. See this documentation of package locks for further explanation.

I've found that there will be a new version of npm 5.7.1 with the new command npm ci, that will install from package-lock.json only

The new npm ci command installs from your lock-file ONLY. If your package.json and your lock-file are out of sync then it will report an error.

It works by throwing away your node_modules and recreating it from scratch.

Beyond guaranteeing you that you'll only get what is in your lock-file it's also much faster (2x-10x!) than npm install when you don't start with a node_modules.

As you may take from the name, we expect it to be a big boon to continuous integration environments. We also expect that folks who do production deploys from git tags will see major gains.

Short Answer:

• npm install honors package-lock.json only if it satisfies the requirements of package.json.
• If it doesn't satisfy those requirements, packages are updated & package-lock is overwritten.
• If you want the install to fail instead of overwriting package-lock when this happens, use npm ci.

Here is a scenario that might explain things (Verified with NPM 6.3.0)

You declare a dependency in package.json like:

"depA": "^1.0.0"

Then you do, npm install which will generate a package-lock.json with:

"depA": "1.0.0"

Few days later, a newer minor version of "depA" is released, say "1.1.0", then the following holds true:

npm ci       # respects only package-lock.json and installs 1.0.0

npm install  # also, respects the package-lock version and keeps 1.0.0 installed 
             # (i.e. when package-lock.json exists, it overrules package.json)

Next, you manually update your package.json to:

"depA": "^1.1.0"

Then rerun:

npm ci      # will try to honor package-lock which says 1.0.0
            # but that does not satisfy package.json requirement of "^1.1.0" 
            # so it would throw an error 

npm install # installs "1.1.0" (as required by the updated package.json)
            # also rewrites package-lock.json version to "1.1.0"
            # (i.e. when package.json is modified, it overrules the package-lock.json)

Use the newly introduced

npm ci

npm ci promises the most benefit to large teams. Giving developers the ability to “sign off” on a package lock promotes more efficient collaboration across large teams, and the ability to install exactly what is in a lockfile has the potential to save tens if not hundreds of developer hours a month, freeing teams up to spend more time building and shipping amazing things.

Introducing npm ci for faster, more reliable builds

Use the npm ci command instead of npm install.

"ci" stands for "continuous integration".

It will install the project dependencies based on the package-lock.json file instead of the lenient package.json file dependencies.

It will produce identical builds to your team mates and it is also much faster.

You can read more about it in this blog post: https://blog.npmjs.org/post/171556855892/introducing-npm-ci-for-faster-more-reliable