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Antonio Ruiz
Asked: 2020-05-30 08:21:57 +0800 CST 2020-05-30 08:21:57 +0800 CST

Loop with XMLHttpRequest not working for me

  • 772

With this loop I am checking if the destination page returns a 1 or nothing (blank page), but it always enters the two IFs , I don't understand it. It's wrong?

I have tested the destination page (const url) in the browser and if I am logged in it gives me 1, but otherwise it stays blank.

var i = 1;
var response = 0;

function myLoop () { 
   setTimeout(function () { 
                const Http = new XMLHttpRequest();
                const url='url/get_session'; //Devuelve 1 o nada (página en blanco)
                Http.open("GET", url);
                Http.send();

                Http.onreadystatechange = (e) => {
                    response = Http.responseText.replace(' ','');
                    if(response == '1'){ console.log('In: '+response); }
                    if(response != '1'){ console.log('Out: '+response);
                        //location.reload();
                    }
                }
      i++;
      if (i < 10000000) {  myLoop(); } 
   }, 5000)
}

myLoop();
javascript

1 Answers

  1. Best Answer

    The problem is that you check the value of the result before it reaches you, so it enters Out first and then In:

    The propertyreadyState value can be one of the following:

    • 0 UNSENT Client created, open()not yet called.
    • 1 OPENED open() has been called.
    • 2 HEADERS_RECEIVED send() has been called, headersand statusthey are reachable.
    • 3 LOADING Downloading; responseTextmay have partial data.
    • 4 DONE Operation completed.

    So what you need is to check that the operation has completed before reading the responseText:

    Http.onreadystatechange = (e) => {
  if (Http.readyState === 4) {
    response = +Http.responseText.replace(' ',''); //el + transforma a número
    if(response === 1){ 
      console.log('In: '+response); 
    } else {
      console.log('Out: '+response);
      location.reload();
    }
  }
}

    Although the simplest solution would be not to use onreadystatechange, but instead onload:

    const xhr = new XMLHttpRequest();
console.log('UNSENT', xhr.readyState); // readyState es 0

xhr.open('GET', '/api', true);
console.log('OPENED', xhr.readyState); // readyState es 1

xhr.onprogress = function () {
    console.log('LOADING', xhr.readyState); // readyState es 3
};

xhr.onload = function () {
    console.log('DONE', xhr.readyState); // readyState es 4
};

xhr.send(null);
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