１．Coding style
（１）nodejs: express での戻り値の渡し方
（２）fetch, XMLHttpRequestでのrequest

２．exprssでの戻り値の渡し方

router.post('/send', async function(req, res) {
     const {client_host} = req.body;

    let code, err;
    [ code, err ] = await to_client.connect(client_host);

    if(code == 500) {
        let rt = JSON.stringify({"error":"ECONNRESET"});
        return res.status(400).end(rt);    
    }
    if(code !== 200) {
        let rt = JSON.stringify({"error":""});
        return res.status(400).end(err);    
    
    res.json({
        err : 0,
        msg : status.document_uuid
    });
}

３．request
(1) fetch　

function evt_send(document_uuid) {

        const params = {
            document_uuid : document_uuid
        };

        const url = '/api/invoice/send';

        const opts = {
            method: 'POST',
            cache: 'no-cache',
            credentials: 'same-origin',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(params)
        };

        let response;
        try {
            response = await fetch( url, opts);
        } catch(err) {
            throw err;
        }

        let res;
        try {
            res = await response.json();
        } catch(err) {
            throw err;
        }

        if(response.status == 200) {
            this.API.refresh();
        } ekse {
            alert("送信できませんでした.\n"+res.err);
        }
}

(2) XMLHttpRequest　

function evt_send(document_uuid) {

        const params = {
            document_uuid : document_uuid
        };

        const ajax = new XMLHttpRequest();
        ajax.open('POST', '/api/invoice/send');
        ajax.setRequestHeader('Content-Type', 'application/json');
        ajax.responseType = "json";
        ajax.send(JSON.stringify(params));

        ajax.onload = () => {

            const res = ajax.response;

            if(ajax.status == 200) {
                this.API.refresh();
            } else {
                alert("送信できませんでした.\n"+res.err);
            }
        }
}