Bdtjtk

I have the following function which makes an ajax request o fetch data from an API.

function getSegements(url) {

    return new Promise((resolve, reject) => {

        request = new XMLHttpRequest();

        request.open('GET', url);

        request.setRequestHeader('Content-Type', 'application/json');



        // request.onload = () => resolve(request.response);

        // request.onerror = () => reject(request.status);



        request.onreadystatechange = function() {



            if (request.readyState === 4)

            { 

                if (request.status === 200)

                {

                    data = JSON.parse(request.response);

                    console.log(data.segements);

                    resolve(data); 

                }

                else

                {

                    reject({status: request.status});

                }

            }

        };

        request.send();

    });

}

calling the function:

getSegements(url).then((data) => {

    //console.log(data);

    //data = JSON.parse(data);

    theWheel = new Winwheel({

        'outerRadius'     : 212,

        'textFontSize'    : 16,

        'textOrientation' : 'horizontal',

        'textAlignment'   : 'outer',

        'numSegments'     : data.no,

        'segments'        : data.segements,

        'animation' :           // Specify the animation to use.

        {

            'type'     : 'spinToStop',

            'duration' : 5,     // Duration in seconds.

            'spins'    : 3,     // Default number of complete spins.

            'callbackFinished' : alertPrize

        }

    });

    theWheel.animation.spins = 9;

    wheelSpinning = false;

})

.catch((err)=>{

    console.log(err);

    alert('Request failed.  Returned status of ' + err.status);

});

When there is a fault in WinWheel's parameter it runs the catch block. Why is running like that? Doesn't it depend on the function (in this case getSegements) if then() is going to run or catch()?

Actually .then takes two arguments, one function called when everything is fine and one that gets called when an error occured in the previous chain. In your case you could write:

 getSegments(url).then(

   data => { new Whinweel() },

   error => console.log(error)

 );

Now using .catch(handler) is actually the same as .then(null, handler), and as stated earlier, the error handler gets called if there was an error in the previous chain, including the previous "then" handler.

then() returns a Promise as well, and uncaught exceptions are propagated through the call chain until catch() is found, and therefore catch() runs for any exception caught in a call chain

new Promise((res, rej) => {

  res()

}).then(() => {

  throw "in then"

}).catch(e => console.log(e))