Javascript 如何将值从后端传递到前端

如下面发布的代码所示，我试图将值access_token从fetchAuthenticationToken传递到Ws.js中的方法fetch. 在fetchAuthenticationToken中，我接收access_token值，然后将该值设置为resolve和onResult.到这里，我可以得到access_token的值.processor个网站中的.then()个

在processor web-service中，我调用.then(resp => {res.send(resp);})以便将access_token的值发送到前端.但是console.info of fetch显示以下内容:

{"msg":"I.xx::fetch:http://localhost:3000/xx/processor/","response":{}}

如图所示为空的response

请让我知道我错过了什么，以及如何将access_token的值从后端传递到前端，从fetchAuthenticationToken到fetch

setup.js:

fetchAuthenticationToken(onResult) {
    return new Promise((resolve,reject) => {
        this.#instance.post("/xx/xx/xx/xx/xx-xx/xx", this.#body, this.#config)
        .then(resp => {
            this.#access_token = resp.data.access_token;
            Object.assign(this.#instance.defaults, {headers: {authorization: `Bearer ${resp.data.access_token}`}})
            resolve(this.#access_token);
            onResult(this.#access_token);
        })
    });
}

routes.js:

router.post('/processor', async (req, res, next) => {
        let accessToken = null;
        const setup = new Setup();
        await setup.fetchAuthenticationToken((result) => {
            accessToken = result;
            res.setHeader('Content-Type', 'application/x-protobuf'); 
        })
        .then(resp => {
            res.send(resp);
        })
    });
    

front-end Ws:

fetch(onResult) {
    return new Promise((resolve, reject) => {
        const ws = endPointsURLs.VUE_NEWSWM_APP_LOCAL_HOST_3000.description + endPointsURLs.VUE_NEWSWM_APP_PATH.description + BackendWssConstants.CONST_WS_SENTINELHUB_ENGINE_PROCESSOR.description;
        fetch(ws, this.#requestOptions)
        .then(response => {
            this.#msg = JSON.stringify({msg:infoTag + 'fetch:' + ws, response: response});
            console.info(this.#msg);
            resolve(response);
            onResult(response);
        })
    });
}