Look up on productMap keys as a set (#25590)

Author: peterbe

lib/page.js

@@ -18,6 +18,12 @@ import getLinkData from './get-link-data.js'
 import getDocumentType from './get-document-type.js'
 import { union } from 'lodash-es'
 
+// We're going to check a lot of pages' "ID" (the first part of
+// the relativePath) against `productMap` to make sure it's valid.
+// To avoid having to do `Object.keys(productMap).includes(id)`
+// every single time, we turn it into a Set once.
+const productMapKeysAsSet = new Set(Object.keys(productMap))
+
 // Wrapper on renderContent() that caches the output depending on the
 // `context` by extracting information about the page's current permalink
 const _renderContentCache = new Map()
@@ -162,10 +168,7 @@ class Page {
 
     // make sure the ID is valid
     if (process.env.NODE_ENV !== 'test') {
-      assert(
-        Object.keys(productMap).includes(id),
-        `page ${this.fullPath} has an invalid product ID: ${id}`
-      )
+      assert(productMapKeysAsSet.has(id), `page ${this.fullPath} has an invalid product ID: ${id}`)
     }
 
     return id