install: add --before date support for time traveling~

doc/misc/npm-config.md

@@ -179,6 +179,22 @@ a non-zero exit code.
 
 What authentication strategy to use with `adduser`/`login`.
 
+### before
+
+* Alias: enjoy-by
+* Default: null
+* Type: Date
+
+If passed to `npm install`, will rebuild the npm tree such that only versions
+that were available **on or before** the `before` time get installed.
+If there's no versions available for the current set of direct dependencies, the
+command will error.
+
+If the requested version is a `dist-tag` and the given tag does not pass the
+`before` filter, the most recent version less than or equal to that tag will
+be used. For example, `foo@latest` might install `foo@1.2` even though `latest`
+is `2.0`.
+
 ### bin-links
 
 * Default: `true`

lib/config/defaults.js

@@ -139,6 +139,7 @@ Object.defineProperty(exports, 'defaults', {get: function () {
     'dry-run': false,
     editor: osenv.editor(),
     'engine-strict': false,
+    'enjoy-by': null,
     force: false,
 
     'fetch-retries': 2,
@@ -279,6 +280,7 @@ exports.types = {
   'dry-run': Boolean,
   editor: String,
   'engine-strict': Boolean,
+  'enjoy-by': [null, Date],
   force: Boolean,
   'fetch-retries': Number,
   'fetch-retry-factor': Number,
@@ -394,6 +396,7 @@ function getLocalAddresses () {
 }
 
 exports.shorthands = {
+  before: ['--enjoy-by'],
   s: ['--loglevel', 'silent'],
   d: ['--loglevel', 'info'],
   dd: ['--loglevel', 'verbose'],

lib/install.js

@@ -703,8 +703,25 @@ Installer.prototype.cloneCurrentTreeToIdealTree = function (cb) {
   validate('F', arguments)
   log.silly('install', 'cloneCurrentTreeToIdealTree')
 
-  this.idealTree = copyTree(this.currentTree)
-  this.idealTree.warnings = []
+  if (npm.config.get('before')) {
+    this.idealTree = {
+      package: this.currentTree.package,
+      path: this.currentTree.path,
+      realpath: this.currentTree.realpath,
+      children: [],
+      requires: [],
+      missingDeps: {},
+      missingDevDeps: {},
+      requiredBy: [],
+      error: this.currentTree.error,
+      warnings: [],
+      isTop: true
+    }
+  } else {
+    this.idealTree = copyTree(this.currentTree)
+    this.idealTree.warnings = []
+  }
+
   cb()
 }