Argument passed into graphql mutation is always undefined

Learning graphql, cannot get mutation to work yet, get lost on the online examples.

import express from 'express'
import cors from 'cors'
import { graphqlHTTP } from 'express-graphql'
import { makeExecutableSchema } from '@graphql-tools/schema'

const app = express()
const port = 4000

// In-memory data store
const data = {
  warriors: [
    { id: '001', name: 'Jaime' },
    { id: '002', name: 'Jorah' },
  ],
}

// Schema
const typeDefs = `
type Warrior {
  id: ID!
  name: String!
}

type Query {
  warriors: [Warrior]
}

type Mutation {
  addUser(name: String!): Warrior!
}
`

// Resolver for warriors
const resolvers = {
  Query: {
    warriors: (obj, args, context) => context.warriors,
  },
  Mutation: {
    //addUser: (parent: unknown, args: {name: String}) => {
    addUser: (name) => {
      console.log('------ 1 ----------');
      const id = String(data.warriors.length + 1);
      const warrior = {id, name};
      console.dir(warrior);
      console.log('------ 3 ----------');
      data.warriors.push(warrior);
      return data.warriros;
    },
  },
}

const executableSchema = makeExecutableSchema({
  typeDefs,
  resolvers,
})

app.use(cors())
app.use(express.json())
app.use(express.urlencoded({ extended: true }))

// Entrypoint
app.use(
  '/graphql',
  graphqlHTTP({
    schema: executableSchema,
    context: data,
    graphiql: true,
  })
)

app.listen(port, () => {
  console.log(`Running a server at https://localhost:${port}`)
})

in https://localhost:4000/graphql
i tried to run this, but the name passed in is always undefined

    mutation {
      addUser(name: "ccc") {
       id   
      }
    }

Any suggestions pls ? Thx !