add Jest suite from vue-cli-3 branch

tests/unit/Labels.spec.js

@@ -0,0 +1,42 @@
+import VueSelect from "../../src/components/Select";
+import { shallowMount } from "@vue/test-utils";
+import { selectWithProps } from "../helpers";
+
+describe("Labels", () => {
+  it("can generate labels using a custom label key", () => {
+    const Select = selectWithProps({
+      options: [{ name: "Foo" }],
+      label: "name",
+      value: { name: "Foo" }
+    });
+    expect(Select.find(".selected-tag").text()).toBe("Foo");
+  });
+
+  it("will console.warn when options contain objects without a valid label key", () => {
+    const spy = jest.spyOn(console, "warn").mockImplementation(() => {});
+    const Select = selectWithProps({
+      options: [{}]
+    });
+
+    Select.vm.open = true;
+    expect(spy).toHaveBeenCalledWith(
+      '[vue-select warn]: Label key "option.label" does not exist in options object {}.' +
+        "\nhttp://sagalbot.github.io/vue-select/#ex-labels"
+    );
+  });
+
+  it("should display a placeholder if the value is empty", () => {
+    const Select = shallowMount(VueSelect, {
+      propsData: {
+        options: ["one"]
+      },
+      attrs: {
+        placeholder: "foo"
+      }
+    });
+
+    expect(Select.vm.searchPlaceholder).toEqual("foo");
+    Select.vm.mutableValue = "one";
+    expect(Select.vm.searchPlaceholder).not.toBeDefined();
+  });
+});