Javascript wait or sleep for response from database [duplicate]

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• How do I return the response from an asynchronous call? 16 answers

I'm trying to write an Amazon Lambda function in Node.js to return elements in a DynamoDB database. I cannot use batchGetItem() because I am querying a secondary index, so I must use query. There are several keys that I want to query over, so I tried to use a for loop to issue multiple query requests and keep concatenating the responses, finally returning the end result. I need a way for the function to wait until all the responses are received before issuing the final callback. Any ideas of how to implement this?

 toReturn = [];
 for (var i in zipCodes){
    var params = {
    TableName: 'hosts',
    IndexName: 'zip-index',
    KeyConditionExpression: 'zip = :hkey',
    ExpressionAttributeValues: {
        ':hkey': zipCodes[i]
      }
    };



    dynamo.query(params, function(err, data) {
       if (err){
        console.log(err);
        callback(null, errResponse);
       } 
       else {
           toReturn = toReturn.concat(data.Items);
        if (i == zipCodes.length-1){
            //trigger some event to send the response
        }

  }


  //wait for all the items to get added to toReturn 
  var response = {
                statusCode: 200,
                headers: {
                "Access-Control-Allow-Origin" : "*" // Required for CORS support to work
                },
                body: JSON.stringify({
                  message: 'Stores retrieved succesfully!',
                  items: toReturn,
                  input: event,
                })
            }
            callback(null, response);

via Jay Vaidya