let promiseArr

axios.interceptors.request.use(config => {
    //set header
    config.headers = {
        "Content-Type":"application/json",
        "Accept":"application/json"
    };
    //
    if (promiseArr[config.url]) {
        promiseArr[config.url]("");
        promiseArr[config.url] = cancel
    } else {
        promiseArr[config.url] = cancel
    }
    return config
}, error => {
    return Promise.reject(error)
});

what does promiseArr config.url mean in this code? isn"t this url a string? why can it be assigned