javascript: getting incorrect variable being set when passed as an argument

Question

I have following code & output. I don't understand why the function is receiving incorrect argument. Please provide pointers.

var tempAccounts = [];
var i;
for (i = 0; i < accountIDs.length; i++) {
    var accountID = accountIDs[i];
    console.log("Log1:" + accountID);
    tempAccounts.push(function(callback){createTempAccount(accountID, callback)})
}

async.parallel(tempAccounts, function (error, results) {
    if (error) {
        sendError(error, "error creating temp acounts", res, logTag);
    } else {
        res.set('Content-Type','application/json');
        res.send(200, {});
    }
});


function createTempAccount(accountID, callback)
{
  console.log("Log2:" + accountID);
  ...
}

Output:

Log1: 1234
Log1: abcd

Log2: 1234
Log2: 1234

What could be the issue here?

The main issue is that this does not provide an [MCVE](https://stackoverflow.com/help/mcve), so I suggest you read that and understand it then update accordingly. You probably actually really mean either [JavaScript closure inside loops – simple practical example](https://stackoverflow.com/questions/750486/javascript-closure-inside-loops-simple-practical-example) or you are expecting "series" when you actually said "parallel". But sorting out which of those you mean is what the MCVE is for. — Neil Lunn, Nov 26 '18 at 04:31
@NeilLunn .. That is the correct answer. Now I am getting expected output. Thank you so much! — GJain, Nov 26 '18 at 04:37
You should preferably use `let` instead of `var` when you're writing code in `NodeJS`. — Kaushik, Nov 26 '18 at 04:50

Mark · Accepted Answer · 2018-11-26T04:39:18.003

You don't need all the extra code to see the problem. It can be reduced to the following snippet. When you defined the function in the loop you capture the a shared variable in a closure. Then later when you call those functions, you get just the one value:

var tempAccounts = [];
let accountIDs = [200, 400]
var i
for ( i = 0; i < accountIDs.length; i++) {
    var accountID = accountIDs[i];
    console.log("Log1:" + accountID);
    tempAccounts.push(function(){return accountID})
}
// all the same value
console.log(tempAccounts.map(i => i()))

The easiest way to deal with this is using let to define a new variable in each iteration of the loop:

var tempAccounts = [];
let accountIDs = [200, 400]
for (let i = 0; i < accountIDs.length; i++) {
    let accountID = accountIDs[i];
    console.log("Log1:" + accountID);
    tempAccounts.push(function(){return accountID})
}

// now you get the unqie values:
console.log(tempAccounts.map(i => i()))

Looks like we have older node.js version. I get this error SyntaxError: Block-scoped declarations (let, const, function, class) not yet supported outside strict mode — GJain, Nov 26 '18 at 04:47
Oh, that's a bummer, @GJain. It makes things so much easier. Maybe it wouldn't be hard to `'use strict'` even if just for specific functions. — Mark, Nov 26 '18 at 04:49

javascript: getting incorrect variable being set when passed as an argument

1 Answers1