Better AuthBA
Better Auth•8mo ago
momoneko

Infer type before initializing betterAuth

I'm trying to create a class that to migrate our system to betterAuth, while keeping the current system in place for the time being.

I know what plugins are going to be enabled, but things like database password, appName and others will be passed in via configuration options

How do I correctly define the type for the authInstance so that I have access to all the plugins?

For example:
import { betterAuth, BetterAuthOptions } from "better-auth";
import { jwt } from "better-auth/plugins";


const authInstance = betterAuth({

  emailAndPassword: {
    enabled: true,
  },
  advanced: {
    cookiePrefix: "pva_",
  },
  plugins: [jwt()],
});

export class AuthService {
  authInstance: ReturnType<typeof betterAuth>;
  constructor(o: BetterAuthOptions) {
    this.authInstance = betterAuth({
      ...o,
      emailAndPassword: {
        enabled: true,
      },
      advanced: {
        cookiePrefix: "pva_",
      },
      plugins: [jwt()],
    });
  }

  getSession(headers: Headers) {
    return this.authInstance.api.getSession({
      headers: headers,
    });
  }
  getJwt(headers: Headers) {
    // this doesn't work
    return this.authInstance.api.getToken({headers});
  }
  getInstancejwt(headers: Headers) {
    // this works
    return authInstance.api.getToken({headers})
  }
}
Solution
I think I got it. Here's the diff of the changes I did 
--- oldAuthClass.ts    2025-05-07 18:11:43
+++ authClass.ts    2025-05-07 18:12:53
@@ -1,5 +1,5 @@
-import { betterAuth, BetterAuthOptions } from "better-auth";
-import { jwt } from "better-auth/plugins";
+import { betterAuth, BetterAuthOptions, UnionToIntersection } from "better-auth";
+import { anonymous, jwt } from "better-auth/plugins";
 
 
 const authInstance = betterAuth({
@@ -12,19 +12,28 @@
   },
   plugins: [jwt()],
 });
+const plugins = [anonymous, jwt];
 
+type Plugins = typeof plugins;
+type PlugInReturnTypes = ReturnType<Plugins[number]>;
+type PluginEndpoints = UnionToIntersection<PlugInReturnTypes['endpoints']>;
+type PluginApis = {
+  api: PluginEndpoints;
+};
+
 export class AuthService {
-  authInstance: ReturnType<typeof betterAuth>;
+  authInstance: ReturnType<typeof betterAuth> & PluginApis;
   constructor(o: BetterAuthOptions) {
     this.authInstance = betterAuth({
       ...o,
       emailAndPassword: {
         enabled: true,
       },
       advanced: {
         cookiePrefix: "pva_",
       },
-      plugins: [jwt()],
+      plugins: plugins.map((plugin) => plugin())
     });
   }


The interesting part would be this 
const plugins = [anonymous, jwt];

type Plugins = typeof plugins;
type PlugInReturnTypes = ReturnType<Plugins[number]>;
type PluginEndpoints = UnionToIntersection<PlugInReturnTypes['endpoints']>;
type PluginApis = {
  api: PluginEndpoints;
};


Have fun 😉
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