如何将Node Promise和http一起使用？

Question

I am trying to learn about Promises in node. I want to be able to use the http.request as a promise. This is what I have learnt so far.

I set up a test case using setTimeout as shown below:

var makePromise = (txt, milli) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {  // fn(cb)
            console.log("I promise to...")
            resolve(txt)
        }, milli)
    });
}

var p = makePromise("Come Home", 3000);
var q = makePromise("Go Shopping", 1000);

var tasks = [p, q];

console.log("Start with ", tasks);

Promise.all(tasks).then((res) => {
    console.log(res);
    console.log("Finish");
})

This works fine. Now I thought that all I would need to do was replace the setTimeout function with the appropriate http.request. But it doesn't seem to be as straightforward as that.

Here is the current attempt at this.

var makePromise = (collection, item) => {
    var options = {
        method: "PUT",
        port: 9000,
        host: "localhost",
        path: `/${collection}/item/${item.Label.itemUUID}`,
        headers: {
            "Accept": "application/json",
            "Content-type": "application/json",
            "Content-length": JSON.stringify(req.body).length
        }
    };
    console.log(JSON.stringify(options));

    var httpPromise = new Promise((resolve, reject) => {
        var vot = http.request(options, (votResponse) => {
            var responseBody = [];
            votResponse.on('data', (chunk) => {
                console.log(`Got ${chunk.length} ${chunk}`);
                responseBody.push(chunk);
            });
            votResponse.on('end', () => {
                console.log("End of http");
                console.log("After web proxy like call - raw data: ", Buffer.concat(responseBody).toString());
                responseBody.json = JSON.parse(Buffer.concat(responseBody).toString());
                resolve(responseBody.json);
            });
            votResponse.on('error', (err) => {
                reject(err);
            });
        })
        vot.write(JSON.stringify(item));
        vot.end();
    });
    return httpPromise;
};

I know there are libraries like "request-promise" that can help, but I am keen to get a deeper understanding of the native operations before using a library. Can anyone point out how I might get this approach to work?

The calling code is shown here:

var collection = req.params.collection;
var patchInstruction = req.body.patchInstruction;
switch (patchInstruction) {
    case "CASCADE DELETE": 
        console.log("CASCADE DELETE on", collection, req.body.item);
        var tasks = [req.body.item].concat(req.body.asSubject).concat(req.body.asObject);
        tasks = tasks.map((e, i, a) => {
            e.Item = "DELETED"
            return makeDeletePromise(collection, e); // a promise to do the http PUT
        });
        console.log("WIP ", tasks);
        Promise.all(tasks).then((res) => {
            console.log("To client --> ",res);
            res.send(res);  
        })
        break;
    default:    
        console.log("No path to ",patchInstruction);
        res.send(409);
        break;
}

The call to port 9000 never happens.

Answer 1

It's always a good idea to end a promise chain with a catch call. Otherwise, any error that happens inside a promise chain would be "swallowed" -- you simply will not see it.

So, any time there is a then there should be a catch .

Promise.all(tasks).then((res) => {
    console.log("To client --> ", res);
    res.send(res);
}).catch((err) => console.log(err));

I believe, an error is somewhere near res.send(res) , because http's res is shadowed by local res . Which is really a responseBody.json , so it has no send method.