feat(gatsby-transform-remark): populate id to headings from markdownAST

packages/gatsby-transformer-remark/src/__tests__/__snapshots__/extend-node.js.snap

@@ -523,6 +523,23 @@ Object {
 }
 `;
 
+exports[`Headings are generated correctly from schema returns null id, value and depth 1`] = `
+Object {
+  "headings": Array [
+    Object {
+      "depth": 1,
+      "id": null,
+      "value": "first title",
+    },
+    Object {
+      "depth": 2,
+      "id": null,
+      "value": "second title",
+    },
+  ],
+}
+`;
+
 exports[`Headings are generated correctly from schema returns value 1`] = `
 Object {
   "headings": Array [

packages/gatsby-transformer-remark/src/__tests__/extend-node.js

@@ -1266,6 +1266,35 @@ describe(`Headings are generated correctly from schema`, () => {
     }
   )
 
+  bootstrapTest(
+    `returns null id, value and depth`,
+    `
+  # first title
+
+  ## second title
+  `,
+    `headings {
+        id
+        value
+        depth
+      }`,
+    node => {
+      expect(node).toMatchSnapshot()
+      expect(node.headings).toEqual([
+        {
+          id: null,
+          value: `first title`,
+          depth: 1,
+        },
+        {
+          id: null,
+          value: `second title`,
+          depth: 2,
+        },
+      ])
+    }
+  )
+
   bootstrapTest(
     `returns value with inlineCode`,
     `

packages/gatsby-transformer-remark/src/create-schema-customization.js

@@ -1,5 +1,6 @@
 const typeDefs = `
   type MarkdownHeading {
+    id: String
     value: String
     depth: Int
   }

packages/gatsby-transformer-remark/src/extend-node-type.js

@@ -22,6 +22,7 @@ const {
 } = require(`./hast-processing`)
 const codeHandler = require(`./code-handler`)
 const { timeToRead } = require(`./utils/time-to-read`)
+const { getHeadingID } = require(`./utils/get-heading-id`)
 
 let fileNodes
 let pluginsCacheStr = ``
@@ -271,6 +272,7 @@ module.exports = (
         const ast = await getAST(markdownNode)
         const headings = select(ast, `heading`).map(heading => {
           return {
+            id: getHeadingID(heading),
             value: mdastToString(heading),
             depth: heading.depth,
           }