Empty values in JSON dict from javascript

I have JavaScript at Front:

var mix = {
    methods: {
        signIn () {
            const username = document.querySelector('#login').value
            const password = document.querySelector('#password').value
      var dict = {}
      dict['username']=username
            this.postData('/api/sign-in/', JSON.stringify(dict))
                .then(({ data, status }) => {
                    location.assign(`/`)
                })
                .catch(() => {
                    alert('Error auth')
                })
        }
    },
    mounted() {
    },
    data() {
        return {}
    }
}

And when I send POST query like
{"username": "aa"}

I receive dict like:
{'{"username":"aa"}': ''}

As you can see value is empty, all data set as key. What can be a problem?
Thx.