feat(pkg): add support to empty bracket syntax

Adds ability to using empty bracket syntax as a shortcut to appending
items to the end of an array when using `npm pkg set`, e.g:

npm pkg set keywords[]=foo

It also adds support to negative indexes, so it's now also possible to
retrieve the last item of an array without knowing an exact index, e.g:

npm pkg get keywords[-1]

Relates to: npm/rfcs#402

docs/content/commands/npm-pkg.md

@@ -98,6 +98,13 @@ Returned values are always in **json** format.
     npm pkg set contributors[0].name='Foo' contributors[0].email='foo@bar.ca'
     ```
 
+    You may also append items to the end of an array using the special
+    empty bracket notation:
+
+    ```bash
+    npm pkg get contributors[].name='Bar' contributors[].email='bar@bar.ca'
+    ```
+
     It's also possible to parse values as json prior to saving them to your
     `package.json` file, for example in order to set a `"private": true`
     property:

lib/utils/queryable.js

@@ -10,6 +10,7 @@ const cleanLeadingDot = str =>
 const parseKeys = (key) => {
   const sqBracketItems = new Set()
   const parseSqBrackets = (str) => {
+    str = str.replace('[]', '[-0]')
     const index = sqBracketsMatcher(str)
 
     // once we find square brackets, we recursively parse all these
@@ -124,8 +125,17 @@ const setter = ({ data, key, value, force }) => {
     const maybeIndex = Number(_key)
     if (!Number.isNaN(maybeIndex)) {
       _key = maybeIndex
+
+      // creates new array in case it's missing
       if (!Object.keys(_data).length)
         _data = []
+
+      // in case it's using a negative index, than calculates the
+      // length of the array minus that negative index, it's also used
+      // as a handy shortcut to empty-bracket-appending syntax since
+      // it converts missing indexes like this: arr[] -> arr[-0]
+      if (_key < 0 || Object.is(_key, -0))
+        _key = (_data.length) + _key
     }
 
     // retrieves the next data object to recursively iterate on,

test/lib/pkg.js

@@ -291,6 +291,38 @@ t.test('set single field', t => {
   })
 })
 
+t.test('push to array syntax', t => {
+  const json = {
+    name: 'foo',
+    version: '1.1.1',
+    keywords: [
+      'foo',
+    ],
+  }
+  npm.localPrefix = t.testdir({
+    'package.json': JSON.stringify(json),
+  })
+
+  pkg.exec(['set', 'keywords[]=bar', 'keywords[]=baz'], err => {
+    if (err)
+      throw err
+
+    t.strictSame(
+      readPackageJson(),
+      {
+        ...json,
+        keywords: [
+          'foo',
+          'bar',
+          'baz',
+        ],
+      },
+      'should append to arrays using empty bracket syntax'
+    )
+    t.end()
+  })
+})
+
 t.test('set multiple fields', t => {
   const json = {
     name: 'foo',

test/lib/utils/queryable.js

@@ -602,6 +602,61 @@ t.test('set arrays', async t => {
     { code: 'EOVERRIDEVALUE' },
     'should throw an override error'
   )
+
+  qqq.set('arr[]', 'c')
+  t.strictSame(
+    qqq.toJSON(),
+    {
+      arr: [
+        'a',
+        'b',
+        'c',
+      ],
+    },
+    'should be able to append to array using empty bracket notation'
+  )
+
+  qqq.set('arr[-2]', 'B')
+  t.strictSame(
+    qqq.toJSON(),
+    {
+      arr: [
+        'a',
+        'B',
+        'c',
+      ],
+    },
+    'should be able to use negative indexes'
+  )
+
+  qqq.set('arr[-1]', 'C')
+  t.strictSame(
+    qqq.toJSON(),
+    {
+      arr: [
+        'a',
+        'B',
+        'C',
+      ],
+    },
+    'should be able to use negative indexes'
+  )
+
+  qqq.set('arr[].foo', 'foo')
+  t.strictSame(
+    qqq.toJSON(),
+    {
+      arr: [
+        'a',
+        'B',
+        'C',
+        {
+          foo: 'foo',
+        },
+      ],
+    },
+    'should be able to append objects to array using empty bracket notation'
+  )
 })
 
 t.test('delete values', async t => {