feat: allow extends to inherit from a module (#865)

BREAKING CHANGE: extends functionality now always loads the JSON provided, rather than reading from a specific key

README.md

@@ -1008,7 +1008,41 @@ $ node test.js
   '$0': 'test.js' }
 ```
 
-Note that a configuration object may extend from a JSON file using the `"extends"` property. When doing so, the `"extends"` value should be a path (relative or absolute) to the extended JSON file.
+### `extends` Keyword
+
+`config` and `pkgConf` can provide the `extends` keyword to
+indicate that configuration should inherit from another location.
+
+The value of extends can be either a relative or absolute path to a JSON
+configuration file, e.g.,
+
+```js
+yargs.config({
+  extends: './configs/a.json',
+  logLevel: 'verbose'
+})
+```
+
+Or, a module can be provided (this is useful for creating functionality like
+  [babel-presets](https://babeljs.io/docs/plugins/)).
+
+**my-library.js**
+
+```js
+yargs.pkgConf('nyc')
+```
+
+**consuming package.json**
+
+```json
+{
+  "nyc": {
+    "extends": "nyc-babel-config"
+  }
+}
+```
+
+Where `nyc-babel-config` is a package that exports configuration in its index.
 
 <a name="conflicts"></a>.conflicts(x, y)
 ----------------------------------------------
@@ -1679,8 +1713,6 @@ as a configuration object.
 `cwd` can optionally be provided, the package.json will be read
 from this location.
 
-Note that a configuration stanza in package.json may extend from an identically keyed stanza in another package.json file using the `"extends"` property. When doing so, the `"extends"` value should be a path (relative or absolute) to the extended package.json file.
-
 .recommendCommands()
 ---------------------------
 

lib/apply-extends.js

@@ -15,22 +15,33 @@ function getPathToDefaultConfig (cwd, pathToExtend) {
   return path.resolve(cwd, pathToExtend)
 }
 
-function applyExtends (config, cwd, subKey) {
+function applyExtends (config, cwd) {
   var defaultConfig = {}
 
   if (config.hasOwnProperty('extends')) {
-    var pathToDefault = getPathToDefaultConfig(cwd, config.extends)
+    if (typeof config.extends !== 'string') return defaultConfig
+    var isPath = /\.json$/.test(config.extends)
+    var pathToDefault = null
+    if (!isPath) {
+      try {
+        pathToDefault = require.resolve(config.extends)
+      } catch (err) {
+        // most likely this simply isn't a module.
+      }
+    } else {
+      pathToDefault = getPathToDefaultConfig(cwd, config.extends)
+    }
+    // maybe the module uses key for some other reason,
+    // err on side of caution.
+    if (!pathToDefault && !isPath) return config
 
     checkForCircularExtends(pathToDefault)
 
     previouslyVisitedConfigs.push(pathToDefault)
-    delete config.extends
 
-    defaultConfig = JSON.parse(fs.readFileSync(pathToDefault, 'utf8'))
-    if (subKey) {
-      defaultConfig = defaultConfig[subKey] || {}
-    }
-    defaultConfig = applyExtends(defaultConfig, path.dirname(pathToDefault), subKey)
+    defaultConfig = isPath ? JSON.parse(fs.readFileSync(pathToDefault, 'utf8')) : require(config.extends)
+    delete config.extends
+    defaultConfig = applyExtends(defaultConfig, path.dirname(pathToDefault))
   }
 
   previouslyVisitedConfigs = []

package.json

@@ -39,7 +39,8 @@
     "rimraf": "^2.5.0",
     "standard": "^8.6.0",
     "standard-version": "^3.0.0",
-    "which": "^1.2.9"
+    "which": "^1.2.9",
+    "yargs-test-extends": "^1.0.1"
   },
   "scripts": {
     "pretest": "standard",

test/fixtures/extends/packageA/package.json

@@ -1,6 +1,6 @@
 {
   "foo": {
     "a": 80,
-    "extends": "../packageB/package.json"
+    "extends": "../packageB/index.json"
   }
-}
+}

test/fixtures/extends/packageB/index.json

@@ -0,0 +1,4 @@
+{
+  "a": 90,
+  "b": "riffiwobbles"
+}

test/yargs.js

@@ -1216,6 +1216,31 @@ describe('yargs dsl tests', function () {
         argv.b.should.equal(22)
         argv.z.should.equal(15)
       })
+
+      // see: https://www.npmjs.com/package/yargs-test-extends
+      it('allows a module to be extended, rather than a JSON file', () => {
+        var argv = yargs()
+          .config({
+            a: 2,
+            extends: 'yargs-test-extends'
+          })
+          .argv
+
+        argv.a.should.equal(2)
+        argv.c.should.equal(201)
+      })
+
+      it('ignores an extends key that does not look like a path or module', () => {
+        var argv = yargs()
+          .config({
+            a: 2,
+            extends: 'batman'
+          })
+          .argv
+
+        argv.a.should.equal(2)
+        argv.extends.should.equal('batman')
+      })
     })
   })
 

yargs.js

@@ -478,7 +478,7 @@ function Yargs (processArgs, cwd, parentRequire) {
 
     // If an object exists in the key, add it to options.configObjects
     if (obj[key] && typeof obj[key] === 'object') {
-      conf = applyExtends(obj[key], path || cwd, key)
+      conf = applyExtends(obj[key], path || cwd)
       options.configObjects = (options.configObjects || []).concat(conf)
     }