Add dataLayerName check before push in event. (#20551)

* Add dataLayerName check before push in event.

* Update browser test after data layer name check change.

Author: dewen

packages/gatsby-plugin-google-tagmanager/src/__tests__/gatsby-browser.js

@@ -69,4 +69,23 @@ describe(`onRouteUpdate`, () => {
 
     expect(window.dataLayer).toHaveLength(1)
   })
+
+  it(`registers new data layer variable if dataLayerName is specified`, () => {
+    const { onRouteUpdate } = getAPI(() => {
+      process.env.NODE_ENV = `production`
+    })
+    const dataLayerName = `fooBarDataLater`
+    window[dataLayerName] = []
+
+    onRouteUpdate(
+      {},
+      {
+        dataLayerName,
+      }
+    )
+
+    jest.runAllTimers()
+
+    expect(window[dataLayerName]).toHaveLength(1)
+  })
 })

packages/gatsby-plugin-google-tagmanager/src/gatsby-browser.js

@@ -5,7 +5,11 @@ exports.onRouteUpdate = (_, pluginOptions) => {
   ) {
     // wrap inside a timeout to ensure the title has properly been changed
     setTimeout(() => {
-      window.dataLayer.push({ event: `gatsby-route-change` })
+      let data = pluginOptions.dataLayerName
+        ? window[pluginOptions.dataLayerName]
+        : window.dataLayer
+
+      data.push({ event: `gatsby-route-change` })
     }, 50)
   }
 }