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

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.

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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.

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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)

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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

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Use the npm ci command instead of npm install.

"ci" stands for "clean install".

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

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It appears this issue is fixed in npm v5.4.2

https://github.com/npm/npm/issues/17979

(Scroll down to the last comment in the thread)

Update

Actually fixed in 5.6.0. There was a cross platform bug in 5.4.2 that was causing the issue to still occur.

https://github.com/npm/npm/issues/18712

Update 2

See my answer here: https://stackoverflow.com/a/53680257/1611058

npm ci is the command you should be using when installing existing projects now.

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In the future, you will be able to use a --from-lock-file (or similar) flag to install only from the package-lock.json without modifying it.

This will be useful for CI, etc. environments where reproducible builds are important.

See https://github.com/npm/npm/issues/18286 for tracking of the feature.

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Probably you should use something like this

npm ci

Instead of using npm install if you don't want to change the version of your package.

According to the official documentation, both npm install and npm ci install the dependencies which are needed for the project.

The main difference is, npm install does install the packages taking packge.json as a reference. Where in the case of npm ci, it does install the packages taking package-lock.json as a reference, making sure every time the exact package is installed.

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You probably have something like:

"typescript":"~2.1.6"

in your package.json which npm updates to the latest minor version, in your case being 2.4.1

Edit: Question from OP But that doesn't explain why "npm install" would change the lock file. Isn't the lock file meant to create a reproducible build? If so, regardless of the semver value, it should still use the same 2.1.6 version. Answer: This is intended to lock down your full dependency tree. Let's say typescript v2.4.1 requires widget ~v1.0.0. When you npm install it grabs widget v1.0.0. Later on your fellow developer (or CI build) does an npm install and gets typescript v2.4.1 but widget has been updated to widget v1.0.1. Now your node module are out of sync. This is what package-lock.json prevents. Or more generally: As an example, consider package A: { "name": "A", "version": "0.1.0", "dependencies": { "B": "<0.1.0" } } package B: { "name": "B", "version": "0.0.1", "dependencies": { "C": "<0.1.0" } } and package C: { "name": "C", "version": "0.0.1" } If these are the only versions of A, B, and C available in the registry, then a normal npm install A will install: A@0.1.0 -- B@0.0.1 -- C@0.0.1 However, if B@0.0.2 is published, then a fresh npm install A will install: A@0.1.0 -- B@0.0.2 -- C@0.0.1 assuming the new version did not modify B's dependencies. Of course, the new version of B could include a new version of C and any number of new dependencies. If such changes are undesirable, the author of A could specify a dependency on B@0.0.1. However, if A's author and B's author are not the same person, there's no way for A's author to say that he or she does not want to pull in newly published versions of C when B hasn't changed at all.

OP Question 2: So let me see if I understand correctly. What you're saying is that the lock file specifies the versions of the secondary dependencies, but still relies on the fuzzy matching of package.json to determine the top-level dependencies. Is that accurate? Answer: No. package-lock locks the entire package tree, including the root packages described in package.json. If typescript is locked at 2.4.1 in your package-lock.json, it should remain that way until it is changed. And lets say tomorrow typescript releases version 2.4.2. If I checkout your branch and run npm install, npm will respect the lockfile and install 2.4.1.

More on package-lock.json:

package-lock.json is automatically generated for any operations where npm modifies either the node_modules tree, or package.json. It describes the exact tree that was generated, such that subsequent installs are able to generate identical trees, regardless of intermediate dependency updates.

This file is intended to be committed into source repositories, and serves various purposes:

Describe a single representation of a dependency tree such that teammates, deployments, and continuous integration are guaranteed to install exactly the same dependencies. Provide a facility for users to "time-travel" to previous states of node_modules without having to commit the directory itself. To facilitate greater visibility of tree changes through readable source control diffs. And optimize the installation process by allowing npm to skip repeated metadata resolutions for previously-installed packages.

https://docs.npmjs.com/files/package-lock.json

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EDIT: the name "lock" is a tricky one, its NPM trying to catch up with Yarn. It isn't a locked file whatsoever. package.json is a user-fixed file, that once "installed" will generate node_modules folder tree and that tree will then be written in package-lock.json. So you see, its the other way around - dependency versions will be pulled from package.json as always, and package-lock.json should be called package-tree.json

(hope this made my answer clearer, after so many downvotes)

A simplistic answer: package.json have your dependencies as usual, while package-lock.json is "an exact, and more importantly reproducible node_modules tree" (taken from npm docs itself).

As for the tricky name, its NPM trying to catch up with Yarn.

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There is an open issue for this on their github page: https://github.com/npm/npm/issues/18712

This issue is most severe when developers are using different operating systems.

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Npm install detects any changes made to package.json file to reflect the dependency list accordingly.

Ex. If user added or removed a new dependency, the build will download or remove the dependencies in the local computer. We can compare this to .m2 repository in java where maven keeps track of pom.xml file constantly to update the dependencies.

package-lock.json is a replica of package.json used at run-time by internal processes, only difference is package-lock.json is read-only to user.

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