fix: Always return JSON for outdated --json

Close: #176

EDIT: Added test, do not set exitStatus to 1 if we're just printing an
empty list as JSON. -- @isaacs

lib/outdated.js

@@ -101,7 +101,10 @@ function outdated (args, silent, cb) {
         return aa[0].path.localeCompare(bb[0].path) ||
           aa[1].localeCompare(bb[1])
       })
-      if (er || silent || list.length === 0) return cb(er, list)
+      if (er || silent ||
+          (list.length === 0 && !opts.json)) {
+        return cb(er, list)
+      }
       if (opts.json) {
         output(makeJSON(list, opts))
       } else if (opts.parseable) {
@@ -129,7 +132,7 @@ function outdated (args, silent, cb) {
         }
         output(table(outTable, tableOpts))
       }
-      process.exitCode = 1
+      process.exitCode = list.length ? 1 : 0
       cb(null, list.map(function (item) { return [item[0].parent.path].concat(item.slice(1, 7)) }))
     })
   }))

test/tap/outdated-json.js

@@ -1,10 +1,7 @@
 var fs = require('graceful-fs')
 var path = require('path')
 
-var mkdirp = require('mkdirp')
 var mr = require('npm-registry-mock')
-var osenv = require('osenv')
-var rimraf = require('rimraf')
 var test = require('tap').test
 
 var common = require('../common-tap.js')
@@ -42,8 +39,6 @@ var expected = {
 }
 
 test('setup', function (t) {
-  cleanup()
-  mkdirp.sync(pkg)
   fs.writeFileSync(
     path.join(pkg, 'package.json'),
     JSON.stringify(json, null, 2)
@@ -92,14 +87,37 @@ test('it should log json data', function (t) {
   )
 })
 
+test('it should log json data even when the list is empty', function (t) {
+  common.npm(
+    [
+      'rm',
+      'request',
+      'underscore'
+    ],
+    EXEC_OPTS,
+    function (er, code, stdout) {
+      t.ifError(er, 'run without error')
+      t.is(code, 0, 'successful exit status')
+      common.npm(
+        [
+          '--registry', common.registry,
+          '--silent',
+          '--json',
+          'outdated'
+        ],
+        EXEC_OPTS,
+        function (er, code, stdout) {
+          t.ifError(er, 'run without error')
+          t.is(code, 0, 'successful exit status')
+          t.same(JSON.parse(stdout), {}, 'got an empty object printed')
+          t.end()
+        }
+      )
+    }
+  )
+})
+
 test('cleanup', function (t) {
   server.close()
-  cleanup()
   t.end()
 })
-
-function cleanup () {
-  // windows fix for locked files
-  process.chdir(osenv.tmpdir())
-  rimraf.sync(pkg)
-}