feat: added new rule table-column-count (#392)

Author: SwetaTanwar

README.md

@@ -86,6 +86,7 @@ export default defineConfig([
 | [`no-missing-label-refs`](./docs/rules/no-missing-label-refs.md) | Disallow missing label references | yes |
 | [`no-multiple-h1`](./docs/rules/no-multiple-h1.md) | Disallow multiple H1 headings in the same document | yes |
 | [`require-alt-text`](./docs/rules/require-alt-text.md) | Require alternative text for images | yes |
+| [`table-column-count`](./docs/rules/table-column-count.md) | Disallow data rows in a GitHub Flavored Markdown table from having more cells than the header row | yes |
 <!-- Rule Table End -->
 
 **Note:** This plugin does not provide formatting rules. We recommend using a source code formatter such as [Prettier](https://prettier.io) for that purpose.

docs/rules/table-column-count.md

@@ -0,0 +1,67 @@
+# table-column-count
+
+Disallow data rows in a GitHub Flavored Markdown table from having more cells than the header row.
+
+## Background
+
+In GitHub Flavored Markdown [tables](https://github.github.com/gfm/#tables-extension-), rows should maintain a consistent number of cells. While variations are sometimes tolerated, data rows having *more* cells than the header can lead to lost data or rendering issues. This rule prevents data rows from exceeding the header's column count.
+
+## Rule Details
+
+> [!IMPORTANT] <!-- eslint-disable-line -- This should be fixed in https://github.com/eslint/markdown/issues/294 -->
+>
+> This rule relies on the `table` AST node, typically available when using a GFM-compatible parser (e.g., `language: "markdown/gfm"`).
+
+This rule is triggered if a data row in a GFM table contains more cells than the header row. It does not flag rows with fewer cells than the header.
+
+Examples of **incorrect** code for this rule:
+
+```markdown
+<!-- eslint markdown/table-column-count: "error" -->
+
+| Head1 | Head2 |
+| ----- | ----- |
+| R1C1  | R1C2  | R2C3  | <!-- This data row has 3 cells, header has 2 -->
+
+| A |
+| - |
+| 1 | 2 | <!-- This data row has 2 cells, header has 1 -->
+```
+
+Examples of **correct** code for this rule:
+
+```markdown
+<!-- eslint markdown/table-column-count: "error" -->
+
+<!-- Standard correct table -->
+| Header | Header |
+| ------ | ------ |
+| Cell   | Cell   |
+| Cell   | Cell   |
+
+<!-- Data row with fewer cells than header (VALID for this rule) -->
+<!-- Rows with fewer cells are valid because they render correctly and no data is lost -->
+| Header | Header | Header |
+| ------ | ------ | ------ |
+| Cell   | Cell   |        |
+
+<!-- Table with some empty cells (VALID for this rule) -->
+<!-- Missing cells are treated as empty and don't cause rendering issues -->
+| Col A | Col B | Col C |
+| ----- | ----- | ----- |
+| 1     |       | 3     |
+| 4     | 5     |
+
+<!-- Single column table -->
+| Single Header |
+| ------------- |
+| Single Cell   |
+```
+
+## When Not To Use It
+
+If you intentionally create Markdown tables where data rows are expected to contain more cells than the header, and you have a specific (perhaps non-standard) processing or rendering pipeline that handles this scenario correctly, you might choose to disable this rule. However, adhering to this rule is recommended for typical GFM rendering and data consistency.
+
+## Prior Art
+
+* [MD056 - table-column-count](https://github.com/DavidAnson/markdownlint/blob/main/doc/Rules.md#md056---table-column-count)

src/rules/table-column-count.js

@@ -0,0 +1,73 @@
+/**
+ * @fileoverview Rule to disallow data rows in a GitHub Flavored Markdown table from having more cells than the header row
+ * @author Sweta Tanwar (@SwetaTanwar)
+ */
+
+//-----------------------------------------------------------------------------
+// Type Definitions
+//-----------------------------------------------------------------------------
+
+/**
+ * @typedef {import("../types.ts").MarkdownRuleDefinition<{ RuleOptions: []; }>} TableColumnCountRuleDefinition
+ */
+
+//-----------------------------------------------------------------------------
+// Rule Definition
+//-----------------------------------------------------------------------------
+
+/** @type {TableColumnCountRuleDefinition} */
+export default {
+	meta: {
+		type: "problem",
+
+		docs: {
+			recommended: true,
+			description:
+				"Disallow data rows in a GitHub Flavored Markdown table from having more cells than the header row",
+			url: "https://github.com/eslint/markdown/blob/main/docs/rules/table-column-count.md",
+		},
+
+		messages: {
+			inconsistentColumnCount:
+				"Table column count mismatch (Expected: {{expectedCells}}, Actual: {{actualCells}}), extra data starting here will be ignored.",
+		},
+	},
+
+	create(context) {
+		return {
+			table(node) {
+				if (node.children.length < 1) {
+					return;
+				}
+
+				const headerRow = node.children[0];
+				const expectedCellsLength = headerRow.children.length;
+
+				for (let i = 1; i < node.children.length; i++) {
+					const currentRow = node.children[i];
+					const actualCellsLength = currentRow.children.length;
+
+					if (actualCellsLength > expectedCellsLength) {
+						const firstExtraCellNode =
+							currentRow.children[expectedCellsLength];
+
+						const lastActualCellNode =
+							currentRow.children[actualCellsLength - 1];
+
+						context.report({
+							loc: {
+								start: firstExtraCellNode.position.start,
+								end: lastActualCellNode.position.end,
+							},
+							messageId: "inconsistentColumnCount",
+							data: {
+								actualCells: String(actualCellsLength),
+								expectedCells: String(expectedCellsLength),
+							},
+						});
+					}
+				}
+			},
+		};
+	},
+};