Adds getCollectionItemIndex universal filter.

src/Filters/GetCollectionItemIndex.js

@@ -0,0 +1,16 @@
+module.exports = function getCollectionItemIndex(collection, page) {
+  if (!page) {
+    page = this.page || this.ctx?.page || this.context?.environments?.page;
+  }
+
+  let j = 0;
+  for (let item of collection) {
+    if (
+      item.inputPath === page.inputPath &&
+      (item.outputPath === page.outputPath || item.url === page.url)
+    ) {
+      return j;
+    }
+    j++;
+  }
+};

src/defaultConfig.js

@@ -3,6 +3,7 @@ const serverlessUrlFilter = require("./Filters/ServerlessUrl");
 const slugFilter = require("./Filters/Slug");
 const slugifyFilter = require("./Filters/Slugify");
 const getLocaleCollectionItem = require("./Filters/GetLocaleCollectionItem");
+const getCollectionItemIndex = require("./Filters/GetCollectionItemIndex");
 
 module.exports = function (config) {
   let templateConfig = this;
@@ -23,6 +24,13 @@ module.exports = function (config) {
 
   config.addFilter("serverlessUrl", serverlessUrlFilter);
 
+  config.addFilter(
+    "getCollectionItemIndex",
+    function (collection, pageOverride) {
+      return getCollectionItemIndex.call(this, collection, pageOverride);
+    }
+  );
+
   config.addFilter(
     "getCollectionItem",
     function (collection, pageOverride, langCode) {

test/GetCollectionItemIndexTest.js

@@ -0,0 +1,30 @@
+const test = require("ava");
+const getCollectionItemIndex = require("../src/Filters/GetCollectionItemIndex");
+
+test("getCollectionItemIndex", (t) => {
+  let first = {
+    inputPath: "hello.md",
+    outputPath: "/hello/",
+  };
+  let second = {
+    inputPath: "hello2.md",
+    outputPath: "/hello2/",
+  };
+  let third = {
+    inputPath: "hello3.md",
+    outputPath: "/hello3/",
+  };
+  let collections = [first, second, third];
+
+  t.deepEqual(getCollectionItemIndex(collections, first), 0);
+  t.deepEqual(getCollectionItemIndex(collections, second), 1);
+  t.deepEqual(getCollectionItemIndex(collections, third), 2);
+
+  t.deepEqual(
+    getCollectionItemIndex(collections, {
+      inputPath: "unknown.md",
+      outputPath: "/unknown/",
+    }),
+    undefined
+  );
+});