NodeJS Async module: how to get results object in the callback?

Question

Code Snippets:

var async = require("async");

async.map([
    "a",
    "b",
    "c"
], function(thing, callback) {
  console.log(thing + "-something");
  callback();
},
function(err, results){
    if (err) return console.error(err);
    console.log(results);
});

My current results:

a-something
b-something
c-something
[ undefined, undefined, undefined ]

My desired results:

[ a-something, b-something, c-something ]

Could you guys tell me what I am getting wrong? How do I have access to the results object in the callback?

go-oleg · Accepted Answer · 2013-11-22 02:27:56Z

4

You need to pass your result into callback() as the second parameter (the first parameter should be the error if there is any):

var async = require("async");

async.map([
    "a",
    "b",
    "c"
], function(thing, callback) {
  var returnValue = thing + "-something";
  console.log(returnValue);
  callback(null, returnValue);
},
function(err, results){
    if (err) return console.error(err);
    console.log(results);
});

answered Nov 22, 2013 at 2:27

go-oleg

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2 Comments

samol Over a year ago

This works! Thanks Oleg. I wanted to bother you with one more question, I am new to javascript and node and I find the documentation to be diffcult to read. How come callback() also works, in the absense of any argument being passed in. Could you refer me to the documentation that shows callback(null, returnValue)

go-oleg Over a year ago

@SidneySidaZhang: This is the section of the docs you should look at: github.com/caolan/async#maparr-iterator-callback. The reason callback() also works is because JavaScript isn't strict with the number of parameters passed to a function (any parameters not passed are treated as undefined). The async library follows suit and chooses to be relaxed about requiring the parameters so if you don't pass anything in, it assumes theres no error, and like you saw, passes undefined to the last callback.

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