解析云代码上的for循环的奇怪行为。

Question

I am running a very basic hello world code on parse cloud code . I put a simple for loop in the cloud code and just to keep track wrote a console.log and printed the value of 'i'. This is how it looks like ..

// Use Parse.Cloud.define to define as many cloud functions as you want.
// For example:
Parse.Cloud.define("hello", function(request, response) {
   for(var i = 0;i<5;i++)
    {
      console.log("Count : " + i);
    }
response.success("Hello world!");
});

Ideally it should print the count sequentially .. But the output in my log is something like this ..

I2014-08-14T08:06:39.751Z] v9: Ran cloud function hello with:
  Input: {}
  Result: Hello world!
I2014-08-14T08:06:39.767Z] Count : 2
I2014-08-14T08:06:39.769Z] Count : 1
I2014-08-14T08:06:39.770Z] Count : 4
I2014-08-14T08:06:39.771Z] Count : 3
I2014-08-14T08:06:39.781Z] Count : 0

i need to call a function in a loop for my implementation. but its not working properly. Is there something wrong with for loop or is there some different way to implement the for loop in parse cloud code. I am clueless. Please help me with this issue. Thanks in advance.

Answer 1

There is nothing wrong. You are just seeing the fact that Parse.com CloudCode and nearly all JavaScript you will encounter runs asynchronously.

console.log doesn't print immediately to the console like some code would running in a shell on your local system. It's more like saying to the system "Please print this console.log message as soon as you can" it's up to the system when that gets executed. All that you know is that it will happen sometime.

Logging isn't that important (and Parse.com console sucks) so there isn't a way to control this behaviour but for operations where ordering is important, you can get a callback when the operation is complete in order to schedule your next operation. Eg

console.log("Count : 0", { success: function() {
    console.log("Count : 1", { success: function() {
            console.log("Count : 2", { success: function() {
        }); 
    }); 
}); 

Note that this is just an example - you can't actually run that code because console.log doesn't support callbacks. Check out Parse.Query which does.

The code can get a little cumbersome and over indented so as you get further into it, you should start to learn how to use promises. See Parse.Promise.