fix(npm-conf): Do not overwrite defaults with undefined cli keys

utils/npm-conf/__tests__/npm-conf.test.js

@@ -29,4 +29,22 @@ describe("@lerna/npm-conf", () => {
       "//npm.example.com/some-api/npm-virtual/"
     );
   });
+
+  it("defaults cli parameter to empty object", () => {
+    const conf = npmConf();
+
+    expect(conf.sources.cli.data).toEqual({});
+  });
+
+  it("overwrites default with cli key", () => {
+    const conf = npmConf({ registry: "https://npm.example.com" });
+
+    expect(conf.get("registry")).toBe("https://npm.example.com");
+  });
+
+  it("does not overwrite default with undefined cli key", () => {
+    const conf = npmConf({ registry: undefined });
+
+    expect(conf.get("registry")).toBe("https://registry.npmjs.org/");
+  });
 });

utils/npm-conf/lib/npm-conf.js

@@ -14,7 +14,19 @@ module.exports.toNerfDart = toNerfDart;
 function npmConf(opts) {
   const conf = new Conf(Object.assign({}, defaults.defaults));
 
-  conf.add(Object.assign({}, opts), "cli");
+  // prevent keys with undefined values from obscuring defaults
+  const cleanOpts = opts
+    ? Object.keys(opts).reduce((acc, key) => {
+        if (opts && opts[key] !== undefined) {
+          // eslint-disable-next-line no-param-reassign
+          acc[key] = opts[key];
+        }
+
+        return acc;
+      }, {})
+    : {};
+
+  conf.add(cleanOpts, "cli");
   conf.addEnv();
   conf.loadPrefix();