fix(package): Add Package.lazy() helper

core/package/__tests__/core-package.test.js

@@ -239,3 +239,35 @@ describe("Package", () => {
     });
   });
 });
+
+describe("Package.lazy()", () => {
+  loadJsonFile.sync.mockImplementation(() => ({ name: "bar", version: "1.0.0" }));
+
+  it("returns package instance from string directory argument", () => {
+    const pkg = Package.lazy("/foo/bar");
+
+    expect(pkg).toBeInstanceOf(Package);
+    expect(pkg.location).toBe("/foo/bar");
+  });
+
+  it("returns package instance from package.json file argument", () => {
+    const pkg = Package.lazy("/foo/bar/package.json");
+
+    expect(pkg).toBeInstanceOf(Package);
+    expect(pkg.location).toBe("/foo/bar");
+  });
+
+  it("returns package instance from json and dir arguments", () => {
+    const pkg = Package.lazy({ name: "bar", version: "1.2.3" }, "/foo/bar");
+
+    expect(pkg).toBeInstanceOf(Package);
+    expect(pkg.version).toBe("1.2.3");
+  });
+
+  it("returns existing package instance", () => {
+    const existing = new Package({ name: "existing" }, "/foo/bar", "/foo");
+    const pkg = Package.lazy(existing);
+
+    expect(pkg).toBe(existing);
+  });
+});

core/package/index.js

@@ -58,6 +58,10 @@ class Package {
         configurable: true,
         value: pkg,
       },
+      // safer than instanceof across module boundaries
+      __isLernaPackage: {
+        value: true,
+      },
       // immutable
       bin: {
         value:
@@ -205,4 +209,22 @@ class Package {
   }
 }
 
+function lazy(ref, dir = ".") {
+  if (typeof ref === "string") {
+    const location = path.resolve(path.basename(ref) === "package.json" ? path.dirname(ref) : ref);
+    const manifest = loadJsonFile.sync(path.join(location, "package.json"));
+
+    return new Package(manifest, location);
+  }
+
+  // don't use instanceof because it fails across nested module boundaries
+  if ("__isLernaPackage" in ref) {
+    return ref;
+  }
+
+  // assume ref is a json object
+  return new Package(ref, dir);
+}
+
 module.exports = Package;
+module.exports.lazy = lazy;