NodeJS Elasticsearch搜索返回promise而不是值

Question

I made a request inside my Node server to Elasticsearch. This is working perfect, except that I always get a promise returned instead of the results.

When I console log the results they look perfect, but when I return them I either get nothing or a promise.

Can someone tell me the proper way to retrieve, and handle the data from Elasticsearch?

I am using VueJS with a Node server and the official Elasticsearch package.

    function getNewTest(client)
    {
        client.search({
            index: 'myIndex',
        }).then(function(resp) {

            return resp.hits.hits;

        }, function(err) {

            console.trace(err.message);

        });
    }

    let tests = getNewTest(client);
    console.log(tests);

    # Output: Promise { <pending> }

EDIT: As suggested I tried both codes, both didnt work. I changed my own code, now it returns an "undefined" to me.

getNewTest(client).then(function (response) {
           console.log(response);
        });

will return "undefined" to me. I changed my function to this:

async function getNewTest(client)
{
    await client.search({
        index: 'myIndex',
    }).then(function(resp) {

        console.log(resp.hits.hits, 'returned');
        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });
}

When I would do

let test = getNewTest(client);

it returns a promise to me.

Answer 1

(async () => {

let tests = await getNewTest(client);
console.log(tests);
})();

You are making a db call, so the main thread becomes free and start executing the next line. The code needs to wait till the promise is resolved and then execute the next line.

Or if you dont want to use async await, you can use this code below -

async function getNewTest(client) {
    client.search({
        index: 'myIndex',
    }).then(function (resp) {

        return resp.hits.hits;

    }, function (err) {

        console.trace(err.message);

    });
}


getNewTest(client).then(result => {
    console.log(result);
});

Answer 2

Function getNewTest will alway return undefined because you do not explicitly return anything.

Even if you did it this way :

function getNewTest(client)
{
    return client.search({
        index: 'myIndex',
    }).then(function(resp) {

        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });
}

It will return a promise.

 const generatePromise = () => new Promise((resolve, reject) => { setTimeout(() => { resolve(1500) }, 500) }) function test () { return generatePromise() .then((data) => console.log('after promise resolved ', data)) .catch((err) => console.log(err)) } console.log('before calling test function'); const result = test() console.log('after calling test function', result instanceof Promise); 

When we call a function that returns a promise (async work) the execution does not wait for the promise to resolve it continue executing other code, that why const result = test() won't have the result of the promise. The result of the promise will only be available inside the then handler as you can see in the code snippet.

 const generatePromise = () => new Promise((resolve, reject) => { setTimeout(() => { resolve(1500) }, 500) }) function test () { return generatePromise() } console.log('before calling test function'); test() .then((data) => console.log('after promise resolved ', data)) .catch((err) => console.log(err)) console.log('after calling test function'); 

You can achieve the required using async & await:

 const generatePromise = () => new Promise((resolve, reject) => { setTimeout(() => { resolve(1500) }, 500) }) // async function always returns promise even if you did't explicitly do async function test () { const data = await generatePromise(); // you can access resolved data here console.log('after promise resolved ', data); return data; } // first without then console.log('before calling test function'); // you can't access it here unless you use then // test().then(data => console.log(data)); const result = test(); console.log('after calling test function', result instanceof Promise); 

This is how asynchronous works you can't return a promise and expect to receive the results instantly you can access the results inside the then handles or use await as I did.

Answer 3

When you do this:

async function getNewTest(client)
{
    await client.search({
        index: 'myIndex',
    }).then(function(resp) {

        console.log(resp.hits.hits, 'returned');
        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });
}

it means this:

async function getNewTest(client)
{
    await client.search({
        index: 'myIndex',
    }).then(function(resp) {

        console.log(resp.hits.hits, 'returned');
        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });

    return undefined; // you are deliberately returning undefined
}

Remember that in javascript if you don't return anything the result of the function is undefined. I'm guessing what you intend to do is:

async function getNewTest(client)
{
    return await client.search({ // NOTE THIS LINE
        index: 'myIndex',
    }).then(function(resp) {

        console.log(resp.hits.hits, 'returned');
        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });
}

Since .search() already returns a Promise (or promise-like object) you don't need to await for it. The code above is exactly the same as:

function getNewTest(client)
{
    return client.search({ // NOTE THIS RETURN
        index: 'myIndex',
    }).then(function(resp) {

        console.log(resp.hits.hits, 'returned');
        return resp.hits.hits;

    }, function(err) {

        console.trace(err.message);

    });
}

But still, this does not allow you to do let test = getNewTest(client) . Nothing will ever make this possible. It is simply impossible . To get the result of getNewTest() either call it's .then() method or await for it. In other words, either do this:

getNewTest(client).then(function(test) { /*continue logic here*/ })

or do this:

async function foo () {
    let test = await getNewTest(client);

    /*continue logic here*/
}

foo();

Note that this mechanism applies everywhere. It is also impossible to do this:

async function foo () {
    let test = await getNewTest(client);
    return test;
}

let test = foo();

You must instead do this if you want to go this route:

async function foo () {
    let test = await getNewTest(client);
    return test;
}

async function bar () {
    let test = await foo();
    /*continue logic here*/
}

There is no escape. You can never directly return an asynchronous value.