bug/#3251_linkStyle-can't-specify-ids Fixed

packages/mermaid/src/diagrams/flowchart/flowDb.js

@@ -192,6 +192,12 @@
  */
 export const updateLink = function (positions, style) {
   positions.forEach(function (pos) {
+    if (pos >= edges.length) {
+      let error = new Error(
+        `The index for linkStyle is out of bounds. (Help: Ensure that the index is within the range of existing edges.)`
+      );
+      throw error;
+    }
     if (pos === 'default') {
       edges.defaultStyle = style;
     } else {

packages/mermaid/src/diagrams/flowchart/parser/flow-style.spec.js

@@ -286,6 +286,30 @@ describe('[Style] when parsing', () => {
     expect(edges[0].type).toBe('arrow_point');
   });
 
+  it('should handle style definitions within number of edges', function () {
+    try {
+      flow.parser.parse(`graph TD
+      A-->B
+      linkStyle 1 stroke-width:1px;`);
+      // Fail test if above expression doesn't throw anything.
+      expect(true).toBe(false);
+    } catch (e) {
+      expect(e.message).toBe(
+        `The index for linkStyle is out of bounds. (Help: Ensure that the index is within the range of existing edges.)`
+      );
+    }
+  });
+
+  it('should handle style definitions within number of edges', function () {
+    const res = flow.parser.parse(`graph TD
+    A-->B
+    linkStyle 0 stroke-width:1px;`);
+
+    const edges = flow.parser.yy.getEdges();
+
+    expect(edges[0].style[0]).toBe('stroke-width:1px');
+  });
+
   it('should handle multi-numbered style definitions with more then 1 digit in a row', function () {
     const res = flow.parser.parse(
       'graph TD\n' +