Debugger pause following Meteor.call, maybe client/server debugging

Having trouble debugging a Meteor method on the server side. While debugging in the client (using Chrome), the debugger progresses until the 4th line of code ("Meteor.call") and then immediately jumps back to the 2nd line ("convlist:function()"), skipping over the 2nd debugger instruction. There are no errors in the client or server consoles. I've also tried using the server-side debugger, but the running process never seems to reach it (Server debug at http://localhost:8080/debug?port=5858). Any advice would be greatly appreciated.

Client Side:

Template.conversationList.helpers({
  convlist: function(){
    debugger;
    Meteor.call('getConvList', function(error, result){
    if(error){
      alert('Error');
    } else {
      debugger; // used for evaluating the result variable
      return result;
    }
  });
  //edited 3rd debugger; 
  debugger;
}});

Server Side:

if (Meteor.isServer) {
  Meteor.methods({
    getConvList: function(){
      debugger;
      let myUser = new Array();
      myUser.push(Meteor.user()._id);
      var newConv = Conversations.aggregate([{ "$match" : { "users": {"$in":                         [Meteor.user()._id]}}}, { "$project": { lstmsg:1, "conversator": {"$setDifference": ["$users", myUser] }}}]);
      return newConv;
    }
  });
}
javascriptmeteor

Answer №1

  1. Why does the server-side method return a value that appears as undefined on the client-side? Explanation: The value returned from the server seems to be undefined when debugging on the client side, but the actual answer comes later from the server. To capture the returned value and receive dynamic updates when the server responds, try using a Session.set variable.
  1. Why does the client debugger stop at line 4?

When the method is called on the client side, it only passes a callback function to be executed later and does not run immediately. The command "debugger" acts as a breakpoint and is not actually executed, so it is expected that the client only reaches line 4 before finishing the convlist function.

  1. Why doesn't the code switch to the server-side debugger?

After starting "meteor debug", the server is initially paused by default. You need to open the node inspector (http://localhost:8080/debug?port=5858) for the server to unpause and launch your app. If using Firefox, note that it may not load the node inspector properly, preventing the server from unpausing and causing the break in the server-side debugging.

  1. Why does the server-side method return a value that appears as undefined on the client-side?

The method call is asynchronous, so when invoked on the client side, it immediately returns undefined and starts executing on the server. Once the execution is complete, the callback function is triggered, providing access to the result value. Store this result in a Session variable to display it in your template once returned from the server method call.

Ensure that the "debugger" instructions are within the callback function passed to the method call. Remember to use the correct method call syntax: Method.call('methodName', inputParameter, callbackFunction); Try passing a null inputParameter and observe the outcome of the callback execution - differences have been noted in this scenario. When no inputParameter is provided, both error and result are undefined during the callback, but with an inputParam, the function receives the proper parameter. For example, on the client side:

Template.conversationList.helpers({
  convlist: function(){
    debugger;
    Meteor.call('getConvList', null, function(error, result){
    if(error){
      alert('Error');
    } else {
      debugger; // evaluate the result var
      return result;
    }
  });
  //edited 3rd debugger; 
  debugger;
}});

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