fix: Preinstall for either npm or yarn.

Closes #15

src/preinstall.test.ts

@@ -0,0 +1,46 @@
+import { execSync } from "child_process";
+import TestUtils from "./test/util";
+import preinstall from "./preinstall";
+
+const packageName = '@joberstein12/commitlint-config';
+
+describe("src/preinstall", () => {
+    const {
+        createTempDirectory,
+        teardownTestDirectory,
+        options,
+    } = new TestUtils();
+
+    beforeEach(() => {
+        options.cwd = createTempDirectory();
+        process.chdir(options.cwd);
+    });
+
+    afterEach(() => {
+        teardownTestDirectory(options.cwd as string);
+    });
+
+    it("Installs for an npm app", () => {
+        execSync('npm init -y && npm install', options);
+
+        preinstall(packageName, options);
+
+        const isInstalled = execSync(`npm list ${packageName} | grep ${packageName}`)
+            .toString()
+            .trim();
+
+        expect(isInstalled).toBeTruthy();
+    });
+
+    it("Installs for a yarn app", () => {
+        execSync('yarn init -y && yarn install', options);
+
+        preinstall(packageName, options);
+
+        const isInstalled = execSync(`yarn list --pattern ${packageName} | grep ${packageName}`)
+            .toString()
+            .trim();
+
+        expect(isInstalled).toBeTruthy();
+    });
+});

src/preinstall.ts

@@ -1,4 +1,5 @@
 import { ExecSyncOptions, execSync } from 'child_process';
+import { existsSync } from 'fs';
 
 /**
  * Pre-install extra dependencies.
@@ -15,6 +16,10 @@ export default (packageString?: string, options?: ExecSyncOptions): void => {
     .join(' ');
 
   if (packages) {
-    execSync(`npm install ${packages} --silent`, options);
+    const command = existsSync('yarn.lock')
+      ? `yarn add ${packages} --silent`
+      : `npm install ${packages} --silent`;
+
+    execSync(command, options);
   }
 };