Allow provided config object to extend other configs

.editorconfig

@@ -0,0 +1,7 @@
+root = true
+
+[*.js]
+end_of_line = lf
+indent_style = space
+indent_size = 2
+insert_final_newline = true

lib/apply-extends.js

@@ -0,0 +1,19 @@
+var fs = require('fs')
+var path = require('path')
+var assign = require('./assign')
+
+function applyExtends (config, cwd) {
+  var defaultConfig = {}
+
+  if (config.hasOwnProperty('extends')) {
+    var pathToDefault = path.join(cwd, config.extends)
+    delete config.extends
+
+    defaultConfig = JSON.parse(fs.readFileSync(pathToDefault, 'utf8'))
+    defaultConfig = applyExtends(defaultConfig, path.dirname(pathToDefault))
+  }
+
+  return assign(defaultConfig, config)
+}
+
+module.exports = applyExtends

test/fixtures/extends/config_1.json

@@ -0,0 +1,5 @@
+{
+  "a": 30,
+  "b": 22,
+  "extends": "./config_2.json"
+}

test/fixtures/extends/config_2.json

@@ -0,0 +1,3 @@
+{
+  "z": 15
+}

test/yargs.js

@@ -1155,6 +1155,21 @@ describe('yargs dsl tests', function () {
       argv.foo.should.equal(1)
       argv.bar.should.equal(2)
     })
+
+    describe('extends', function () {
+      it('applies default configurations when given config object', function () {
+        var argv = yargs
+            .config({
+              extends: './test/fixtures/extends/config_1.json',
+              a: 1
+            })
+          .argv
+
+        argv.a.should.equal(1)
+        argv.b.should.equal(22)
+        argv.z.should.equal(15)
+      })
+    })
   })
 
   describe('normalize', function () {

yargs.js

@@ -9,6 +9,7 @@ const Validation = require('./lib/validation')
 const Y18n = require('y18n')
 const objFilter = require('./lib/obj-filter')
 const setBlocking = require('set-blocking')
+const applyExtends = require('./lib/apply-extends')
 const YError = require('./lib/yerror')
 
 var exports = module.exports = Yargs
@@ -304,6 +305,7 @@ function Yargs (processArgs, cwd, parentRequire) {
     argsert('[object|string] [string|function] [function]', [key, msg, parseFn], arguments.length)
     // allow a config object to be provided directly.
     if (typeof key === 'object') {
+      key = applyExtends(key, cwd)
       options.configObjects = (options.configObjects || []).concat(key)
       return self
     }
@@ -319,6 +321,7 @@ function Yargs (processArgs, cwd, parentRequire) {
     ;(Array.isArray(key) ? key : [key]).forEach(function (k) {
       options.config[k] = parseFn || true
     })
+
     return self
   }