loopback 3 setTimeout strange behavior

Question

I'm confused by this kind of strange behavior from loopback. I created a very simple remote method as an example below.

Device.getTypes = function(next) {
    let result = {0: {val: 10}};
    setTimeout(function() {
        result[0].wait = true;
    }, 10);
    console.log(result); // {'0': {val: 10}}
    return next(null, result);
};

The console prints out exactly what I expected. result[0].wait is not defined in this context. But responded JSON data really got me confused.

{
    "data":{
        "0":{
            "val": 10,
            "wait": true
        }
    }
}

I don't understand why "wait": true was there. I then increased delay time to 100. data[0].wait was gone. Does loopback perform a delay in the back?

Answer 1

This behaviour is nothing to do with loopback, it is a node.js feature. node.js in asynchronous in nature(and if you want a synchronous code, you need to use some tricks such as Promise, async libraries, ...). but, that's not just it. console.log is an exception and it is a blocking i/o call, and in your case when your setTimeout is called with a value less than the blocking time, you get "wait": true in your result, otherwise it returns the result before making that change.