fix: save existing anon tokens for multiple init calls (#562)

### Context

In InstantSearch, in order for user tokens to be set in time for the
first query on page load, anonymous user tokens are generated and set as
both the InstantSearch token and Insights token.

This flow has one issue though. When `useCookies` flag needs to be
changed later (such as with a cookie consent banner), a second `init`
call to insights with the flag set to true does not save the existing
token to a cookie.

A short repro is as follows with the very latest version of
InstantSearch where this behaviour was added (PR:
algolia/instantsearch#6377):

```js
const searchClient = algoliasearch(
  // ...
);

window.aa('init', {
  appId: 'id',
  apiKey: 'key',
});

const search = instantsearch({
  indexName: 'index',
  searchClient,
  insights: true,
});

search.addWidgets([
  // ...
]);

// This does not save the cookie atm
setTimeout(() => {
  window.aa('init', {
    partial: true,
    useCookie: true,
  });
}, 5000);
```

lib/__tests__/init.test.ts

@@ -1,6 +1,7 @@
 import { getCookie, MONTH } from "../_tokenUtils";
 import AlgoliaAnalytics from "../insights";
 import * as utils from "../utils";
+import { createUUID } from "../utils/uuid";
 
 jest.mock("../utils", () => ({
   __esModule: true,
@@ -240,6 +241,23 @@ describe("init", () => {
 
     setUserToken.mockRestore();
   });
+  it("should save anonymous userToken as cookie when useCookie is set to true later", () => {
+    analyticsInstance.init({
+      apiKey: "***",
+      appId: "XXX"
+    });
+
+    analyticsInstance.setUserToken(`anonymous-${createUUID()}`);
+
+    analyticsInstance.init({
+      partial: true,
+      useCookie: true
+    });
+
+    expect(document.cookie).toEqual(
+      expect.stringMatching(/^_ALGOLIA=anonymous-/)
+    );
+  });
   it("should replace existing options when called again", () => {
     analyticsInstance.init({
       apiKey: "apiKey1",

lib/_tokenUtils.ts

@@ -31,6 +31,24 @@ export const getCookie = (name: string): string => {
   return "";
 };
 
+export function checkIfAnonymousToken(token: number | string): boolean {
+  if (typeof token === "number") {
+    return false;
+  }
+
+  return token.indexOf("anonymous-") === 0;
+}
+
+export function saveTokenAsCookie(this: AlgoliaAnalytics): void {
+  const foundToken = getCookie(COOKIE_KEY);
+  if (
+    this._userToken &&
+    (!foundToken || foundToken === "" || foundToken.indexOf("anonymous-") !== 0)
+  ) {
+    setCookie(COOKIE_KEY, this._userToken, this._cookieDuration);
+  }
+}
+
 export function setAnonymousUserToken(
   this: AlgoliaAnalytics,
   inMemory = false

lib/init.ts

@@ -1,5 +1,5 @@
 import { DEFAULT_ALGOLIA_AGENTS } from "./_algoliaAgent";
-import { MONTH } from "./_tokenUtils";
+import { checkIfAnonymousToken, MONTH } from "./_tokenUtils";
 import type AlgoliaAnalytics from "./insights";
 import { isUndefined, isNumber } from "./utils";
 
@@ -80,6 +80,8 @@ You can visit https://algolia.com/events/debugger instead.`);
     this.setUserToken(options.userToken);
   } else if (!this._userToken && !this._userHasOptedOut && this._useCookie) {
     this.setAnonymousUserToken();
+  } else if (checkIfTokenNeedsToBeSaved(this)) {
+    this.saveTokenAsCookie();
   }
 }
 
@@ -110,3 +112,15 @@ function setOptions(
     )
   );
 }
+
+function checkIfTokenNeedsToBeSaved(target: AlgoliaAnalytics): boolean {
+  if (target._userToken === undefined) {
+    return false;
+  }
+
+  return (
+    checkIfAnonymousToken(target._userToken) &&
+    target._useCookie &&
+    !target._userHasOptedOut
+  );
+}

lib/insights.ts

@@ -11,7 +11,8 @@ import {
   MONTH,
   setAuthenticatedUserToken,
   onAuthenticatedUserTokenChange,
-  getAuthenticatedUserToken
+  getAuthenticatedUserToken,
+  saveTokenAsCookie
 } from "./_tokenUtils";
 import {
   clickedObjectIDsAfterSearch,
@@ -78,6 +79,7 @@ class AlgoliaAnalytics {
   getVersion: typeof getVersion;
   addAlgoliaAgent: typeof addAlgoliaAgent;
 
+  saveTokenAsCookie: typeof saveTokenAsCookie;
   setUserToken: typeof setUserToken;
   setAnonymousUserToken: typeof setAnonymousUserToken;
   getUserToken: typeof getUserToken;
@@ -110,6 +112,7 @@ class AlgoliaAnalytics {
 
     this.addAlgoliaAgent = addAlgoliaAgent.bind(this);
 
+    this.saveTokenAsCookie = saveTokenAsCookie.bind(this);
     this.setUserToken = setUserToken.bind(this);
     this.setAnonymousUserToken = setAnonymousUserToken.bind(this);
     this.getUserToken = getUserToken.bind(this);