Add smoke test for js users

package.json

@@ -11,6 +11,9 @@
     "prepretest": "rimraf ./coverage ./.nyc_output",
     "pretest": "tsc -p tsconfig.test.json",
     "test": "nyc ts-mocha ./test/test.js",
+    "pretest-js": "rimraf ./test-js-dir",
+    "test-js": "node test-js",
+    "posttest-js": "rimraf ./test-js-dir",
     "posttest": "rimraf ./test",
     "prepublishOnly": "npm run lint && npm run build"
   },

test-js.js

@@ -0,0 +1,28 @@
+const promisify = require('util').promisify;
+const fs = require('fs');
+const exec = promisify(require('child_process').exec);
+const mkdir = promisify(fs.mkdir);
+const writeFile = promisify(fs.writeFile);
+const assert = require('assert');
+
+const code = `
+const update = require('immutability-helper');
+
+const state1 = ['x'];
+const state2 = update(state1, {$push: ['y']})
+console.log(JSON.stringify(state2));
+`
+
+Promise.resolve()
+  .then(() => mkdir('test-js-dir'))
+  .then(() => process.chdir('test-js-dir'))
+  .then(() => exec('npm init -y'))
+  .then(() => exec('npm install ../'))
+  .then(() => writeFile('foo.js', code))
+  .then(() => exec('node foo'))
+  .then(result => assert.strictEqual(result.stdout.trim(), '["x","y"]'))
+  .then(() => process.exit(0))
+  .catch(error => {
+    console.error('uncaught error', error);
+    process.exit(1);
+  });