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website/node_modules/npm/lib/install/is-only-dev.js

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+'use strict'
+module.exports = isOnlyDev
+
+const moduleName = require('../utils/module-name.js')
+const isDevDep = require('./is-dev-dep.js')
+const isProdDep = require('./is-prod-dep.js')
+
+// Returns true if the module `node` is only required direcctly as a dev
+// dependency of the top level or transitively _from_ top level dev
+// dependencies.
+// Dual mode modules (that are both dev AND prod) should return false.
+function isOnlyDev (node, seen) {
+  if (!seen) seen = new Set()
+  return node.requiredBy.length && node.requiredBy.every(andIsOnlyDev(moduleName(node), seen))
+}
+
+// There is a known limitation with this implementation: If a dependency is
+// ONLY required by cycles that are detached from the top level then it will
+// ultimately return true.
+//
+// This is ok though: We don't allow shrinkwraps with extraneous deps and
+// these situation is caught by the extraneous checker before we get here.
+function andIsOnlyDev (name, seen) {
+  return function (req) {
+    const isDev = isDevDep(req, name)
+    const isProd = isProdDep(req, name)
+    if (req.isTop) {
+      return isDev && !isProd
+    } else {
+      if (seen.has(req)) return true
+      seen.add(req)
+      return isOnlyDev(req, seen)
+    }
+  }
+}