fix(uninstall): use correct local prefix

Corrects non-global prefix path by using this.npm.localPrefix. A
location already used throughout the repository. Later, this will be
taken from the unified config library.

Signed-off-by: James Chen-Smith <jameschensmith@gmail.com>

PR-URL: #2930
Credit: @jameschensmith
Close: #2930
Reviewed-by: @ruyadorno

lib/uninstall.js

@@ -38,8 +38,9 @@ class Uninstall extends BaseCommand {
   async uninstall (args) {
     // the /path/to/node_modules/..
     const global = this.npm.config.get('global')
-    const prefix = this.npm.config.get('prefix')
-    const path = global ? resolve(this.npm.globalDir, '..') : prefix
+    const path = global
+      ? resolve(this.npm.globalDir, '..')
+      : this.npm.localPrefix
 
     if (!args.length) {
       if (!global)

test/lib/uninstall.js

@@ -22,6 +22,7 @@ const uninstall = new Uninstall(npm)
 t.afterEach(cb => {
   npm.globalDir = ''
   npm.prefix = ''
+  npm.localPrefix = ''
   npm.flatOptions.global = false
   npm.flatOptions.prefix = ''
   cb()
@@ -85,7 +86,7 @@ t.test('remove single installed lib', t => {
   const b = resolve(path, 'node_modules/b')
   t.ok(() => fs.statSync(b))
 
-  npm.config.set('prefix', path)
+  npm.localPrefix = path
 
   uninstall.exec(['b'], err => {
     if (err)
@@ -148,7 +149,7 @@ t.test('remove multiple installed libs', t => {
   t.ok(() => fs.statSync(a))
   t.ok(() => fs.statSync(b))
 
-  npm.config.set('prefix', path)
+  npm.localPrefix = path
 
   uninstall.exec(['b'], err => {
     if (err)
@@ -196,7 +197,6 @@ t.test('no args global', t => {
   npm.localPrefix = resolve(path, 'projects', 'a')
   npm.globalDir = resolve(path, 'lib', 'node_modules')
   npm.config.set('global', true)
-  npm.config.set('prefix', path)
 
   const a = resolve(path, 'lib/node_modules/a')
   t.ok(() => fs.statSync(a))