fix: fix passing a single Node to mermaid.init()

Passing a single Node to mermaid.init() results in an error, as it
calls `new NodeList()`, which causes `TypeError: Illegal constructor`.

See 5597cf45bf/src/mermaid.ts (L73)

If we instead use the `ArrayLike` interface, we can just use a simple
array, instead of a NodeList.

I've also added a basic test case, by mocking the `mermaidAPI.render()`
function so it isn't called, as the d3 functions don't work in Node.JS.
The mocks are a bit messy, since
 a) Jest doesn't fully support ESM yet, and
 b) mermaidAPI is frozen with `Object.freeze()`,
but the mermaidAPI mocks work as long as we keep them simple.

Fixes: c68ec54fdd

src/__mocks__/mermaidAPI.ts

@@ -0,0 +1,47 @@
+/**
+ * Mocks for `./mermaidAPI`.
+ * 
+ * We can't easily use `jest.spyOn(mermaidAPI, "function")` since the object
+ * is frozen with `Object.freeze()`.
+ */
+import * as configApi from '../config';
+
+import { addDiagrams } from '../diagram-api/diagram-orchestration';
+import Diagram from '../Diagram';
+
+// Normally, we could just do the following to get the original `parse()`
+// implementation, however, requireActual isn't currently supported in Jest
+// for ESM, see https://github.com/facebook/jest/issues/9430
+// and https://github.com/facebook/jest/pull/10976
+// const {parse} = jest.requireActual("./mermaidAPI");
+
+let hasLoadedDiagrams = false;
+function parse(text: string, parseError?: Function): boolean {
+  if (!hasLoadedDiagrams) {
+    addDiagrams();
+    hasLoadedDiagrams = true;
+  }
+  const diagram = new Diagram(text, parseError);
+  return diagram.parse(text, parseError);
+}
+
+// original version cannot be modified since it was frozen with `Object.freeze()`
+export const mermaidAPI = {
+  render: jest.fn(),
+  parse,
+  parseDirective: jest.fn(),
+  initialize: jest.fn(),
+  getConfig: configApi.getConfig,
+  setConfig: configApi.setConfig,
+  getSiteConfig: configApi.getSiteConfig,
+  updateSiteConfig: configApi.updateSiteConfig,
+  reset: () => {
+    configApi.reset();
+  },
+  globalReset: () => {
+    configApi.reset(configApi.defaultConfig);
+  },
+  defaultConfig: configApi.defaultConfig,
+}
+
+export default mermaidAPI;

src/mermaid.spec.js

@@ -1,4 +1,5 @@
 import mermaid from './mermaid';
+import { mermaidAPI } from './mermaidAPI';
 import flowDb from './diagrams/flowchart/flowDb';
 import flowParser from './diagrams/flowchart/parser/flow';
 import flowRenderer from './diagrams/flowchart/flowRenderer';
@@ -6,6 +7,13 @@ import Diagram from './Diagram';
 
 const spyOn = jest.spyOn;
 
+// mocks the mermaidAPI.render function (see `./__mocks__/mermaidAPI`)
+jest.mock('./mermaidAPI');
+
+afterEach(() => {
+  jest.restoreAllMocks();
+});
+
 describe('when using mermaid and ', function () {
   describe('when detecting chart type ', function () {
     it('should not start rendering with mermaid.startOnLoad set to false', function () {
@@ -40,6 +48,16 @@ describe('when using mermaid and ', function () {
     });
   });
 
+  describe('when using #initThrowsErrors', function () {
+    it('should accept single node', async () => {
+      const node = document.createElement('div');
+      node.appendChild(document.createTextNode('graph TD;\na;'));
+
+      mermaid.initThrowsErrors(undefined, node);
+      expect(mermaidAPI.render).toHaveBeenCalled();
+    });
+  });
+
   describe('when calling addEdges ', function () {
     beforeEach(function () {
       flowParser.parser.yy = flowDb;

src/mermaid.ts

@@ -64,14 +64,13 @@ const initThrowsErrors = function (
 
   // if last argument is a function this is the callback function
   log.debug(`${!callback ? 'No ' : ''}Callback function found`);
-  let nodesToProcess: NodeListOf<HTMLElement>;
+  let nodesToProcess: ArrayLike<HTMLElement>;
   if (typeof nodes === 'undefined') {
     nodesToProcess = document.querySelectorAll('.mermaid');
   } else if (typeof nodes === 'string') {
     nodesToProcess = document.querySelectorAll(nodes);
   } else if (nodes instanceof HTMLElement) {
-    nodesToProcess = new NodeList() as NodeListOf<HTMLElement>;
-    nodesToProcess[0] = nodes;
+    nodesToProcess = [nodes];
   } else if (nodes instanceof NodeList) {
     nodesToProcess = nodes;
   } else {