In version 5, npm introduced the package-lock.json file.

What’s that? You probably know about the package.json file, which is much more common and has been around for much longer.

The goal of the file is to keep track of the exact version of every package that is installed so that a product is 100% reproducible in the same way even if packages are updated by their maintainers.

This solves a very specific problem that package.json left unsolved. In package.json you can set which versions you want to upgrade to (patch or minor), using the semver notation, for example:

• if you write ~0.13.0, you want to only update patch releases: 0.13.1 is ok, but 0.14.0 is not.
• if you write ^0.13.0, you want to update patch and minor releases: 0.13.1, 0.14.0 and so on.
• if you write 0.13.0, that is the exact version that will be used, always

You don’t commit to Git your node_modules folder, which is generally huge, and when you try to replicate the project on another machine by using the npm install command, if you specified the ~ syntax and a patch release of a package has been released, that one is going to be installed. Same for ^ and minor releases.

If you specify exact versions, like 0.13.0 in the example, you are not affected by this problem.

It could be you, or another person trying to initialize the project on the other side of the world by running npm install.

So your original project and the newly initialized project are actually different. Even if a patch or minor release should not introduce breaking changes, we all know bugs can (and so, they will) slide in.

The package-lock.json sets your currently installed version of each package in stone, and npm will use those exact versions when running npm install.

This concept is not new, and other programming languages package managers (like Composer in PHP) use a similar system for years.

The package-lock.json file needs to be committed to your Git repository, so it can be fetched by other people, if the project is public or you have collaborators, or if you use Git as a source for deployments.

The dependencies versions will be updated in the package-lock.json file when you run npm update.

## An example

This is an example structure of a package-lock.json file we get when we run npm install cowsay in an empty folder:

{
"requires": true,
"lockfileVersion": 1,
"dependencies": {
"ansi-regex": {
"version": "3.0.0",
"resolved": "https://registry.npmjs.org/ansi-regex/-/ansi-regex-3.
0.0.tgz",
"integrity": "sha1-7QMXwyIGT3lGbAKWa922Bas32Zg="
},
"cowsay": {
"version": "1.3.1",
"resolved": "https://registry.npmjs.org/cowsay/-/cowsay-1.3.1.tgz"
,
"integrity": "sha512-3PVFe6FePVtPj1HTeLin9v8WyLl+VmM1l1H/5P+BTTDkM
Ajufp+0F9eLjzRnOHzVAYeIYFF5po5NjRrgefnRMQ==",
"requires": {
"get-stdin": "^5.0.1",
"optimist": "~0.6.1",
"string-width": "~2.1.1",
"strip-eof": "^1.0.0"
}
},
"get-stdin": {
"version": "5.0.1",
"resolved": "https://registry.npmjs.org/get-stdin/-/get-stdin-5.0.
1.tgz",
"integrity": "sha1-Ei4WFZHiH/TFJTAwVpPyDmOTo5g="
},
"is-fullwidth-code-point": {
"version": "2.0.0",
"resolved": "https://registry.npmjs.org/is-fullwidth-code-point/-/
is-fullwidth-code-point-2.0.0.tgz",
"integrity": "sha1-o7MKXE8ZkYMWeqq5O+764937ZU8="
},
"minimist": {
"version": "0.0.10",
"resolved": "https://registry.npmjs.org/minimist/-/minimist-0.0.10
.tgz",
"integrity": "sha1-3j+YVD2/lggr5IrRoMfNqDYwHc8="
},
"optimist": {
"version": "0.6.1",
"resolved": "https://registry.npmjs.org/optimist/-/optimist-0.6.1.tgz",
"integrity": "sha1-2j6nRob6IaGaERwybpDrFaAZZoY=",

"requires": {
"minimist": "~0.0.1",
"wordwrap": "~0.0.2"
}
},
"string-width": {
"version": "2.1.1",
"resolved": "https://registry.npmjs.org/string-width/-/string-width-2.1.1.tgz",
"integrity": "sha512-nOqH59deCq9SRHlxq1Aw85Jnt4w6KvLKqWVik6oA9ZklXLNIOlqg4F2yrT1MVaTjAqvVwdfeZ7w7aCvJD7ugkw==",
"requires": {
"is-fullwidth-code-point": "^2.0.0",
"strip-ansi": "^4.0.0"
}
},
"strip-ansi": {
"version": "4.0.0",
"resolved": "https://registry.npmjs.org/strip-ansi/-/strip-ansi-4.0.0.tgz",
"integrity": "sha1-qEeQIusaw2iocTibY1JixQXuNo8=",
"requires": {
"ansi-regex": "^3.0.0"
}
},
"strip-eof": {
"version": "1.0.0",
"resolved": "https://registry.npmjs.org/strip-eof/-/strip-eof-1.0.0.tgz",
"integrity": "sha1-u0P/VZim6wXYm1n80SnJgzE2Br8="
},
"wordwrap": {
"version": "0.0.3",
"resolved": "https://registry.npmjs.org/wordwrap/-/wordwrap-0.0.3.tgz",
"integrity": "sha1-o9XabNXAvAAI03I0u68b7WMFkQc="
}
}
}

We installed cowsay, which depends on

• get-stdin
• optimist
• string-width
• strip-eof

In turn, those packages require other packages, as we can see from the requires property that some have:

• ansi-regex
• is-fullwidth-code-point
• minimist
• wordwrap
• strip-eof

They are added in alphabetical order into the file, and each one has a version field, a resolved field that points to the package location, and an integrity string that we can use to verify the package.