Allow registered diagrams to be overridden.

packages/mermaid/src/diagram-api/detectType.ts

@@ -71,10 +71,9 @@
 
 export const addDetector = (key: string, detector: DiagramDetector, loader?: DiagramLoader) => {
   if (detectors[key]) {
-    log.error(`Detector with key ${key} already exists`);
-  } else {
-    detectors[key] = { detector, loader };
+    log.warn(`Detector with key ${key} already exists. Overwriting.`);
   }
+  detectors[key] = { detector, loader };
   log.debug(`Detector with key ${key} added${loader ? ' with loader' : ''}`);
 };
 

packages/mermaid/src/diagram-api/diagramAPI.ts

@@ -49,7 +49,7 @@
   detector?: DiagramDetector
 ) => {
   if (diagrams[id]) {
-    throw new Error(`Diagram ${id} already registered.`);
+    log.warn(`Diagram with id ${id} already registered. Overwriting.`);
   }
   diagrams[id] = diagram;
   if (detector) {

packages/mermaid/src/diagram.spec.ts

@@ -2,9 +2,32 @@ import { describe, test, expect } from 'vitest';
 import { Diagram, getDiagramFromText } from './Diagram.js';
 import { addDetector } from './diagram-api/detectType.js';
 import { addDiagrams } from './diagram-api/diagram-orchestration.js';
+import type { DiagramLoader } from './diagram-api/types.js';
 
 addDiagrams();
 
+const getDummyDiagram = (id: string, title?: string): Awaited<ReturnType<DiagramLoader>> => {
+  return {
+    id,
+    diagram: {
+      db: {
+        getDiagramTitle: () => title ?? id,
+      },
+      parser: {
+        parse: () => {
+          // no-op
+        },
+      },
+      renderer: {
+        draw: () => {
+          // no-op
+        },
+      },
+      styles: {},
+    },
+  };
+};
+
 describe('diagram detection', () => {
   test('should detect inbuilt diagrams', async () => {
     const graph = (await getDiagramFromText('graph TD; A-->B')) as Diagram;
@@ -21,30 +44,25 @@ describe('diagram detection', () => {
     addDetector(
       'loki',
       (str) => str.startsWith('loki'),
-      () =>
-        Promise.resolve({
-          id: 'loki',
-          diagram: {
-            db: {},
-            parser: {
-              parse: () => {
-                // no-op
-              },
-            },
-            renderer: {
-              draw: () => {
-                // no-op
-              },
-            },
-            styles: {},
-          },
-        })
+      () => Promise.resolve(getDummyDiagram('loki'))
     );
-    const diagram = (await getDiagramFromText('loki TD; A-->B')) as Diagram;
+    const diagram = await getDiagramFromText('loki TD; A-->B');
     expect(diagram).toBeInstanceOf(Diagram);
     expect(diagram.type).toBe('loki');
   });
 
+  test('should allow external diagrams to override internal ones with same ID', async () => {
+    const title = 'overridden';
+    addDetector(
+      'flowchart-elk',
+      (str) => str.startsWith('flowchart-elk'),
+      () => Promise.resolve(getDummyDiagram('flowchart-elk', title))
+    );
+    const diagram = await getDiagramFromText('flowchart-elk TD; A-->B');
+    expect(diagram).toBeInstanceOf(Diagram);
+    expect(diagram.db.getDiagramTitle?.()).toBe(title);
+  });
+
   test('should throw the right error for incorrect diagram', async () => {
     await expect(getDiagramFromText('graph TD; A-->')).rejects.toThrowErrorMatchingInlineSnapshot(`
       "Parse error on line 2:

packages/mermaid/src/mermaid.ts

@@ -15,6 +15,7 @@ import { isDetailedError } from './utils.js';
 import type { DetailedError } from './utils.js';
 import type { ExternalDiagramDefinition } from './diagram-api/types.js';
 import type { UnknownDiagramError } from './errors.js';
+import { addDiagrams } from './diagram-api/diagram-orchestration.js';
 
 export type {
   MermaidConfig,
@@ -243,6 +244,7 @@ const registerExternalDiagrams = async (
     lazyLoad?: boolean;
   } = {}
 ) => {
+  addDiagrams();
   registerLazyLoadedDiagrams(...diagrams);
   if (lazyLoad === false) {
     await loadRegisteredDiagrams();