scratch-config-file-for-sj.diff

diff --git a/src/config.js b/src/config.js
new file mode 100644
index 0000000..226fd0c
--- /dev/null
+++ b/src/config.js
@@ -0,0 +1,6 @@
+export function applyConfigToArgv({argv, configObject}) {
+  // Replace argv with config file option values.
+  // This is of course only a naiive implementation that is missing
+  // many things.
+  return {...argv, ...configObject};
+}
diff --git a/tests/unit/test.config.js b/tests/unit/test.config.js
new file mode 100644
index 0000000..35b2097
--- /dev/null
+++ b/tests/unit/test.config.js
@@ -0,0 +1,70 @@
+import {assert} from 'chai';
+import {describe, it} from 'mocha';
+import sinon from 'sinon';
+
+import {fake} from './helpers';
+import {Program} from '../../src/program';
+import {applyConfigToArgv} from '../../src/config';
+
+function makeArgv({
+  userCmd = [],
+  command = 'fakecommand',
+  commandDesc = 'this is a fake command',
+  commandExecutor = sinon.stub(),
+  commandOpt,
+  globalOpt,
+}) {
+  const program = new Program(userCmd)
+  if (globalOpt) {
+    program.setGlobalOptions(globalOpt);
+  }
+  if (commandOpt) {
+    program.command(command, commandDesc, commandExecutor, commandOpt);
+  }
+
+  return program.yargs.exitProcess(false).argv;
+}
+
+describe('config', () => {
+  describe('applyConfigToArgv', () => {
+    it('overrides a string option value with a configured value', () => {
+
+      const argv = makeArgv({
+        globalOpt: {
+          'source-dir': {
+            requiresArg: true,
+            type: 'string',
+            demand: false,
+          },
+        },
+      });
+
+      const configObject = {
+        sourceDir: '/configured/source/dir',
+      };
+      const newArgv = applyConfigToArgv({argv, configObject});
+      assert.strictEqual(newArgv.sourceDir, configObject.sourceDir);
+    });
+
+    it('preserves a string option value on the command line', () => {
+      const cmdLineSrcDir = '/user/specified/source/dir/';
+
+      const argv = makeArgv({
+        userCmd: ['fakecommand', '--source-dir', cmdLineSrcDir],
+        globalOpt: {
+          'source-dir': {
+            requiresArg: true,
+            type: 'string',
+            demand: false,
+          },
+        },
+      });
+
+      const configObject = {
+        sourceDir: '/configured/source/dir',
+      };
+      const newArgv = applyConfigToArgv({argv, configObject});
+      assert.strictEqual(newArgv.sourceDir, cmdLineSrcDir);
+    });
+  });
+});

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