Firebase gives 'appCheck is not a function' when using the client SDK in version 8

Question

I'm getting an error when I initialize the AppCheck in my project.

Firebase version: "^8.8.1".

import "firebase/functions";

const firebaseConfig = {
    apiKey: "--",
    authDomain: "--",
    projectId: "--",
    storageBucket: "--",
    messagingSenderId: "--",
    appId: "--"
};

const app = firebase.initializeApp(firebaseConfig)

const check = firebase.appCheck() // here is the error
check.activate('appCheckKey', true)

const functions = firebase.functions()

export {
    app,
    functions
}

console error:

Uncaught (in promise) TypeError: firebase.appCheck is not a function