feat: add isDefault method

Add a new method `isDefault(key)` that enables a way to know if the
value to be returned by a `config.get(key)` call is coming from the
default definitions or any other different source. In case it's coming
from the default values this method returns `true`, it returns `false`
otherwise.

This addition is going to allow for effectively managing different
default values in the npm cli on a command-basis, e.g:

The `save` config default value in the npm cli is `true`, so that in
`npm install` and other commands that value is always going to default
to true if no user config is provided. Now let's say we want to use a
different value in `npm update`, for example `save=false`. This change
enables us to have a conditional check to see if the `save` config value
is coming from the default source, thus providing a way to use a
different value instead of the default:

    const save = config.isDefault('save') ? false : config.get('save')

Relates to: npm/cli#4223
Relates to: npm/statusboard#324

README.md

@@ -218,6 +218,29 @@ Note that it's usually enough (and more efficient) to just check
 `config.valid`, since each data object is marked for re-evaluation on every
 `config.set()` operation.
 
+### `config.isDefault(key)`
+
+Returns `true` if the value is coming directly from the source defined
+in the default definitions, if the current value for the key config is
+coming from any other source, returns `false`.
+
+Can be useful for avoiding or tweaking default values, e.g:
+
+>  Given a global default definition of foo='foo' it's possible to read that
+>  value such as:
+>
+>  ```js
+>     const save = config.get('foo')
+>  ```
+>
+>  Now in a different place of your app it's possible to avoid using the `foo`
+>  default value, by checking to see if the current config value is currently
+>  one that was defined by the default definitions:
+>
+>  ```js
+>     const save = config.isDefault('foo') ? 'bar' : config.get('foo')
+>  ```
+
 ### `config.save(where)`
 
 Save the config file specified by the `where` param.  Must be one of

lib/index.js

@@ -401,6 +401,20 @@ class Config {
     }
   }
 
+  // Returns true if the value is coming directly from the source defined
+  // in default definitions, if the current value for the key config is
+  // coming from any other different source, returns false
+  isDefault (key) {
+    const [defaultType, ...types] = [...confTypes]
+    const defaultData = this.data.get(defaultType).data
+
+    return hasOwnProperty(defaultData, key)
+      && types.every(type => {
+        const typeData = this.data.get(type).data
+        return !hasOwnProperty(typeData, key)
+      })
+  }
+
   invalidHandler (k, val, type, source, where) {
     this.log.warn(
       'invalid config',

test/index.js

@@ -362,6 +362,33 @@ loglevel = yolo
     ])
     t.equal(config.valid, false)
     logs.length = 0
+
+    // set a new value that defaults to cli source
+    config.set('cli-config', 1)
+
+    t.ok(config.isDefault('methane'),
+      'should return true if value is retrieved from default definitions')
+    t.notOk(config.isDefault('cli-config'),
+      'should return false for a cli-defined value')
+    t.notOk(config.isDefault('foo'),
+      'should return false for a env-defined value')
+    t.notOk(config.isDefault('project-config'),
+      'should return false for a project-defined value')
+    t.notOk(config.isDefault('default-user-config-in-home'),
+      'should return false for a user-defined value')
+    t.notOk(config.isDefault('global-config'),
+      'should return false for a global-defined value')
+    t.notOk(config.isDefault('builtin-config'),
+      'should return false for a builtin-defined value')
+
+    // make sure isDefault still works as intended after
+    // setting and deleting values in differente sources
+    config.set('methane', 'H2O', 'cli')
+    t.notOk(config.isDefault('methane'),
+      'should no longer return true now that a cli value was defined')
+    config.delete('methane', 'cli')
+    t.ok(config.isDefault('methane'),
+      'should return true once again now that values is retrieved from defaults')
   })
 
   t.test('do not double-load project/user config', async t => {