Is Array.forEach in Node.js asynchronous?

Question

Is forEach on an array async ? candies is an array of candy objects.

app.get('/api/:id',function(req, res){

  console.log("Get candy");
  var id = req.params.id;

  candies.forEach( function(candy, index){
    if(candy.id == id){
      console.log("Candy found. Before return");
      return res.json(candy);
      console.log("Candy found. After return");
    }
  });

  console.log("Print error message");
  return res.json({error: "Candy not found"});
});

In the console I get

[nodemon] starting `node app.js`
listning on port 3000
Get candy
Candy found. Before return
Print error message
Error: Can't set headers after they are sent.
   at ServerResponse.setHeader (_http_outgoing.js:367:11)
   ....

Is this a recent change ? It's been awhile since I have done node.js

If it were async, you'd have Print error message logged first. Why would it be async? Also, Thilo pointed out correctly - what's the point of code after return statement? That will never be executed. — Mjh
– Mjh, Commented Feb 28, 2017 at 9:27
Also, the return in the inner function will only exit the inner function, not the outer one. — Thilo
– Thilo, Commented Feb 28, 2017 at 9:27
Ahh there we go.. Of course! Perfect that explains it. I must not be thinking straight today. Thanks @Thilo ! — Jens
– Jens, Commented Feb 28, 2017 at 11:42

GPX · Accepted Answer · 2017-02-28 09:28:39Z

1

You're getting the Can't set headers after they are sent. exception because you're trying to return a response twice - (possibly) once inside candies.forEach and once again in the last line of the route. Also note that any code after a return isn't executed anyways.

Here's how you rewrite it to avoid the error —

app.get('/api/:id',function(req, res){

    console.log("Get candy");
    var id = req.params.id;
    var foundCandy = false;
    candies.forEach( function(candy, index){
        if(candy.id == id){
            foundCandy = true;
            console.log("Candy found. Before return");
        }
    });

    if (foundCandy) {
        return res.json(candy);
    } else {
        return res.json({error: "Candy not found"});
    }
});

answered Feb 28, 2017 at 9:28

GPX

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3 Comments

GPX Over a year ago

@Jens Using Array.filter like @Vladu Ionut's response here also leads to cleaner & more concise code.

Jens Over a year ago

Thanks! I know how to solve it. I am just curious as to why I am getting this behavior. Both res.json(candy) and res.json({error: "Candy not found"}); get's called. That does not seem logical to me unless forEach is now an async function

Jens Over a year ago

Got it... See @Thilo reply

Vladu Ionut · Accepted Answer · 2017-02-28 09:32:08Z

1

You can use Array.filter to find the candy .

app.get('/api/:id', function(req, res) {

  console.log("Get candy");
  var id = req.params.id;

  var result = candies.filter(candy => candy.id == id);

  if (result.length) {
    return res.json(result[0]);
  } else {
    console.log("Print error message");
    return res.json({
      error: "Candy not found"
    });
  }
});

answered Feb 28, 2017 at 9:32

Vladu Ionut

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1 Comment

Jens Over a year ago

To anyone else looking at this question: The "return res.json(candy);" Only returns from the inner function defined in the forEach argument, it does not return from the entire function. There for both responses get called.

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