为什么我的异步 function 不等待 promise 完成

Question

I am using ldapjs to query users from an ldap server. If I put all the code just in a single script without using functions, the query works and I get the results I need.

I am now trying to use expressjs to serve a rest endpoint to enable querying of the ldap server, so I moved the ldapjs client.search code into a async function with a promise surrounding the actual search code. After the promise code, I have a line which exercises the promise using await and stores the results of the promise in a variable. I then return that variable to the calling function which will eventually send the results back as a json-formatted string to the requesting browser.

The problem I am seeing is that the console.log() of the returned results is undefined and appears before the console.log statements inside the promise code. So it looks like the async function is returning before the promise is fulfilled, but I don't see why because in all the examples of promises and async/await I have seen this scenario works correctly.

Below is a sample script without the expressjs part to just make sure everything works correctly.

// script constants:
const ldap = require('ldapjs');
const assert = require('assert');
const ldapServer = "ldap.example.com";
const adSuffix = "dc=example,dc=com"; // test.com
const client = getClient();

const fullName = "*doe*";

var opts = {
  scope: "sub",
  filter: `(cn=${fullName})`,
  attributes: ["displayName", "mail", "title", "manager"]
};
console.log("performing the search");
let ldapUsers = doSearch(client, opts);
console.log("Final Results: " + ldapUsers);

function getClient() {
  // Setup the connection to the ldap server
  ...
  return client;
}

async function doSearch(client, searchOptions) {
  console.log("Inside doSearch()");
  let promise = new Promise((resolve, reject) => {
    users = '{"users": [';
    client.search(adSuffix, searchOptions, (err, res) => {
      if (err) {
        console.log(err);
        reject(err)
      }
      res.on('searchEntry', function(entry) {
        console.log("Entry: " + users.length);
        if (users.length > 11) {
          users = users + "," + JSON.stringify(entry.object);
        } else {
          users = users + JSON.stringify(entry.object);
        }
      });

      res.on('error', function(err) {
        console.error("Error: " + err.message);
        reject(err)
      });
      res.on('end', function(result) {
        console.log("end:");
        client.unbind();
        users = users + "]}";
        resolve(users)
      });
    });
  });


  // resolve the promise:
  let result = await promise;
  console.log("After promise has resolved.");
  console.log(result);
  return result
}

The output from the console.log statements is as follows:

Setting up the ldap client.
ldap.createClient succeeded.
performing the search
Inside doSearch()
Final Results: [object Promise]
Entry: 11
end:
After promise has resolved.
{"users": [{"dn":"cn=john_doe"}]}

I did strip out the code which creates the ldapjs client and redacted the company name, but otherwise this is my code.

Any ideas on why the doSearch function is returning before the promise is fulfilled would be greatly appreciated.

Answer 1

As @danh mentioned in a comment, you're not await ing the response from doSearch . Since doSearch is an async function it will always return a promise, and thus must be await ed.

As a quick and dirty way to do that you could wrap your call in an immediately invoked asynchronous function like so:

// ...

(async () => console.log(await doSearch(client, opts)))();

// ...

For more info you might check out the MDN docs on asynchronous functions

Answer 2

I think there are a few issues in the provided code snippet. As @danh pointed out you need to await the doSearch call. However you may have not done that already since you may not be using an environment with a top async . This likely means you'll want to wrap the call to doSearch in an async function and call that. Assuming you need to await for the search results.

// script constants:
const ldap = require('ldapjs');
const assert = require('assert');
const ldapServer = "ldap.example.com";
const adSuffix = "dc=example,dc=com"; // test.com
const client = getClient();

const fullName = "*doe*";

function getClient() {
  // Setup the connection to the ldap server
  ...
  return client;
}

async function doSearch(client, searchOptions) {
  console.log("Inside doSearch()");
  return new Promise((resolve, reject) => {
    users = '{"users": [';
    client.search(adSuffix, searchOptions, (err, res) => {
      if (err) {
        console.log(err);
        reject(err)
      }
      res.on('searchEntry', function(entry) {
        console.log("Entry: " + users.length);
        if (users.length > 11) {
          users = users + "," + JSON.stringify(entry.object);
        } else {
          users = users + JSON.stringify(entry.object);
        }
      });

      res.on('error', function(err) {
        console.error("Error: " + err.message);
        reject(err)
      });
      res.on('end', function(result) {
        console.log("end:");
        client.unbind();
        users = users + "]}";
        console.log(result);
        resolve(users)
      });
    });
  });
}

const opts = {
  scope: "sub",
  filter: `(cn=${fullName})`,
  attributes: ["displayName", "mail", "title", "manager"]
};

(async function runAsyncSearch () {
  console.log("performing the search");
  try {
    const ldapUsers = await doSearch(client, opts); // Await the async results
    console.log("After promise has resolved.");
    console.log("Final Results: " + ldapUsers);
  } catch (err) {
    console.error(err.message);
  } 
})();  // Execute the function immediately after defining it.