feat: add npm diffruyadorno/npm-diff

- As proposed in RFC: https://github.com/npm/rfcs/pull/144

PR-URL: https://github.com/npm/cli/pull/1319
Credit: @ruyadorno
Close: #1319
Reviewed-by: @isaacs

2 files changed, 294 insertions, 0 deletions

diff --git a/docs/content/commands/npm-diff.md b/docs/content/commands/npm-diff.md
new file mode 100644
index 000000000..0ce5e8dc8
--- /dev/null
+++ b/docs/content/commands/npm-diff.md
@@ -0,0 +1,237 @@
+---
+title: npm-diff
+section: 1
+description: The registry diff command
+---
+
+### Synopsis
+
+```bash
+npm diff [...<paths>]
+npm diff --diff=<pkg-name> [...<paths>]
+npm diff --diff=<version-a> [--diff=<version-b>] [...<paths>]
+npm diff --diff=<spec-a> [--diff=<spec-b>] [...<paths>]
+npm diff [--diff-ignore-all-space] [--diff-name-only] [...<paths>]
+```
+
+### Description
+
+Similar to its `git diff` counterpart, this command will print diff patches
+of files for packages published to the npm registry.
+
+* `npm diff --diff=<spec-a> --diff=<spec-b>`
+
+    Compares two package versions using their registry specifiers, e.g:
+    `npm diff --diff=pkg@1.0.0 --diff=pkg@^2.0.0`. It's also possible to
+    compare across forks of any package,
+    e.g: `npm diff --diff=pkg@1.0.0 --diff=pkg-fork@1.0.0`.
+
+    Any valid spec can be used, so that it's also possible to compare
+    directories or git repositories,
+    e.g: `npm diff --diff=pkg@latest --diff=./packages/pkg`
+
+    Here's an example comparing two different versions of a package named
+    `abbrev` from the registry:
+
+    ```bash
+    npm diff --diff=abbrev@1.1.0 --diff=abbrev@1.1.1
+    ```
+
+    On success, output looks like:
+
+    ```bash
+    diff --git a/package.json b/package.json
+    index v1.1.0..v1.1.1 100644
+    --- a/package.json
+    +++ b/package.json
+    @@ -1,6 +1,6 @@
+     {
+       "name": "abbrev",
+    -  "version": "1.1.0",
+    +  "version": "1.1.1",
+       "description": "Like ruby's abbrev module, but in js",
+       "author": "Isaac Z. Schlueter <i@izs.me>",
+       "main": "abbrev.js",
+    ```
+
+    Given the flexible nature of npm specs, you can also target local
+    directories or git repos just like when using `npm install`:
+
+    ```bash
+    npm diff --diff=https://github.com/npm/libnpmdiff --diff=./local-path
+    ```
+
+    In the example above we can compare the contents from the package installed
+    from the git repo at `github.com/npm/libnpmdiff` with the contents of the
+    `./local-path` that contains a valid package, such as a modified copy of
+    the original.
+
+* `npm diff` (in a package directory, no arguments):
+
+    If the package is published to the registry, `npm diff` will fetch the
+    tarball version tagged as `latest` (this value can be configured using the
+    `tag` option) and proceed to compare the contents of files present in that
+    tarball, with the current files in your local file system.
+
+    This workflow provides a handy way for package authors to see what
+    package-tracked files have been changed in comparison with the latest
+    published version of that package.
+
+* `npm diff --diff=<pkg-name>` (in a package directory):
+
+    When using a single package name (with no version or tag specifier) as an
+    argument, `npm diff` will work in a similar way to
+    [`npm-outdated`](npm-outdated) and reach for the registry to figure out
+    what current published version of the package named <pkg-name> will satisfy
+    its dependent declared semver-range. Once that specific version is known
+    `npm diff` will print diff patches comparing the current version of
+    <pkg-name> found in the local file system with that specific version
+    returned by the registry.
+
+    Given a package named `abbrev` that is currently installed:
+
+    ```bash
+    npm diff --diff=abbrev
+    ```
+
+    That will request from the registry its most up to date version and
+    will print a diff output comparing the currently installed version to this
+    newer one if the version numbers are not the same.
+
+* `npm diff --diff=<spec-a>` (in a package directory):
+
+    Similar to using only a single package name, it's also possible to declare
+    a full registry specifier version if you wish to compare the local version
+    of an installed package with the specific version/tag/semver-range provided
+    in `<spec-a>`.
+
+    An example: assuming `pkg@1.0.0` is installed in the current `node_modules`
+    folder, running:
+
+    ```bash
+    npm diff --diff=pkg@2.0.0
+    ```
+
+    It will effectively be an alias to
+    `npm diff --diff=pkg@1.0.0 --diff=pkg@2.0.0`.
+
+* `npm diff --diff=<semver-a> [--diff=<semver-b>]` (in a package directory):
+
+    Using `npm diff` along with semver-valid version numbers is a shorthand
+    to compare different versions of the current package.
+
+    It needs to be run from a package directory, such that for a package named
+    `pkg` running `npm diff --diff=1.0.0 --diff=1.0.1` is the same as running
+    `npm diff --diff=pkg@1.0.0 --diff=pkg@1.0.1`.
+
+    If only a single argument `<version-a>` is provided, then the current local
+    file system is going to be compared against that version.
+
+    Here's an example comparing two specific versions (published to the
+    configured registry) of the current project directory:
+
+    ```bash
+    npm diff --diff=1.0.0 --diff=1.1.0
+    ```
+
+Note that tag names are not valid `--diff` argument values, if you wish to
+compare to a published tag, you must use the `pkg@tagname` syntax.
+
+#### Filtering files
+
+It's possible to also specify positional arguments using file names or globs
+pattern matching in order to limit the result of diff patches to only a subset
+of files for a given package, e.g:
+
+  ```bash
+  npm diff --diff=pkg@2 ./lib/ CHANGELOG.md
+  ```
+
+In the example above the diff output is only going to print contents of files
+located within the folder `./lib/` and changed lines of code within the
+`CHANGELOG.md` file.
+
+### Configuration
+
+#### diff
+
+* Type: Array<String>
+* Default: null
+
+Defines npm package specifiers to compare using the `npm diff` command.
+
+This can be specified up to 2 times.
+
+#### diff-name-only
+
+* Type: Boolean
+* Default: false
+
+When set to `true` running `npm diff` only returns the names of the files that
+have any difference.
+
+#### diff-unified
+
+* Type: Number
+* Default: `3`
+
+The number of lines of context to print in the unified diff format output.
+
+#### diff-ignore-all-space
+
+* Type: Boolean
+* Default: false
+
+Ignore whitespace when comparing lines. This ignores differences even if one
+line has whitespace where the other line has none.
+
+#### diff-no-prefix
+
+* Type: Boolean
+* Default: false
+
+Do not show any source or destination prefix.
+
+#### diff-src-prefix
+
+* Type: String
+* Default: `"a/"`
+
+Show the given source prefix in diff patches headers instead of using "a/".
+
+#### diff-dst-prefix
+
+* Type: String
+* Default: `"b/"`
+
+Show the given source prefix in diff patches headers instead of using "b/".
+
+#### diff-text
+
+* Type: Boolean
+* Default: false
+
+Treat all files as text.
+
+#### global
+
+* Default: false
+* Type: Boolean
+
+Uses packages from the global space as a source for comparison.
+
+#### tag
+
+* Type: String
+* Default: `"latest"`
+
+The tag used to fetch the tarball that will be compared with the local file
+system files when running npm diff with no arguments.
+
+
+## See Also
+
+* [npm outdated](/commands/npm-outdated)
+* [npm install](/commands/npm-install)
+* [npm config](/commands/npm-config)
+* [npm registry](/using-npm/registry)
diff --git a/docs/content/using-npm/config.md b/docs/content/using-npm/config.md
index 3a2517d34..1e0b20359 100644
--- a/docs/content/using-npm/config.md
+++ b/docs/content/using-npm/config.md
@@ -379,6 +379,63 @@ commands that modify your local installation, eg, `install`, `update`,
 `dedupe`, `uninstall`.  This is NOT currently honored by some network related
 commands, eg `dist-tags`, `owner`, etc.
 
+#### diff
+
+* Default: null
+* Type: String, Array, null
+
+Define arguments to compare in `npm diff`.
+
+#### diff-name-only
+
+* Default: false
+* Type: Boolean
+
+Prints only filenames when using `npm diff`.
+
+#### diff-unified
+
+* Type: number
+* Default: `3`
+
+The number of lines of context to print in `npm diff`.
+
+#### diff-ignore-all-space
+
+* Type: Boolean
+* Default: false
+
+Ignore whitespace when comparing lines in `npm diff.
+
+#### diff-no-prefix
+
+* Type: Boolean
+* Default: false
+
+Do not show any source or destination prefix in `npm diff` output.
+
+#### diff-src-prefix
+
+* Type: String
+* Default: `"a/"`
+
+Source prefix to be used in `npm diff` output.
+
+#### diff-dst-prefix
+
+* Type: String
+* Default: `"b/"`
+
+Destination prefix to be used in `npm diff` output.
+
+#### diff-text
+
+* Alias: `-a`
+* Type: Boolean
+* Default: false
+
+Treat all files as text in `npm diff`.
+
 #### editor
 
 * Default: `EDITOR` environment variable if set, or `"vi"` on Posix,