The XMLHttpRequest function successfully logs data in the console, but does not return the data within the function itself

Question

The XMLHttpRequest function successfully logs data in the console, but does not return the data within the function itself

I find it puzzling that the console.log statement at the end of the code snippet displays the data as expected, while the return optiondata line does not.

function populate_selectbox()
{               
var ajaxRequest;

try {
    // Opera 8.0+, Firefox, Safari
    ajaxRequest = new XMLHttpRequest();
} catch (e) {
    // Internet Explorer Browsers
    try {
        ajaxRequest = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (e) {
        try {
            ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
        } catch (e) {
        // Something went wrong
            alert("Your browser broke!");
            return false;
        }
    }
}

var queryString = "?callFunction=get_all_data";
//console.log(queryString);

ajaxRequest.open("GET", "php/shares.php" + queryString, true);
ajaxRequest.send(null);

var optiondata ="";

// Create a function that will receive data sent from the server
ajaxRequest.onreadystatechange = function() 
{

    if (ajaxRequest.readyState == 4 && ajaxRequest.status == 200) 
    {
        //console.log("IF....");

        var resp = JSON.parse(ajaxRequest.responseText);
        //console.log(resp)

        for (var i = 0; i < resp.data.length; i++) 
        {
            if(i === resp.data.length -1)
            {
                optiondata += "'"+resp.data[i].name+"'"; //For the last name in the loop
            }
            else
            {
                optiondata += "'"+resp.data[i].name+"',";
            }

        }

        console.log(optiondata); //This works
        return optiondata; //This returns undefines
    }    
};  
}

Could you explain why there is a discrepancy in the output?

javascript function xmlhttprequest return

Answer 1

Answer №1

When dealing with ajax callbacks, remember that you cannot return any data using the "return" keyword due to the asynchronous nature of ajax requests.

Instead, create your own callback function and pass the results into it.

function myAjaxFn(..., done) {
    // ...
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
          done(xmlhttp.responseText);
        }
      }
    xmlhttp.open("GET","ajax_info.txt",true);
    xmlhttp.send();
}

Alternatively, consider using promises as a more efficient solution:

Answer 2

When dealing with ajax callbacks, remember that you cannot return any data using the "return" keyword due to the asynchronous nature of ajax requests.

Instead, create your own callback function and pass the results into it.

function myAjaxFn(..., done) {
    // ...
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
          done(xmlhttp.responseText);
        }
      }
    xmlhttp.open("GET","ajax_info.txt",true);
    xmlhttp.send();
}

Alternatively, consider using promises as a more efficient solution:

Answer 3

Answer №2

Consider using the Promise object for asynchronous operations.

    var queryString = "?callFunction=get_all_data";
    var request = function(queryString) {
      return new Promise(function(resolve, reject) {
        var ajaxRequest = new XMLHttpRequest()
        , optiondata = "";
        ajaxRequest.open("GET", "php/shares.php" + queryString, true);
        ajaxRequest.onload = function() {
          if (this.readyState === 4 && this.status === 200) {
            // do something
            var resp = JSON.parse(this.responseText);
            console.log(resp);
            for (var i = 0; i < resp.data.length; i++) {
              if (i === resp.data.length -1) {
                optiondata += "'"+resp.data[i].name+"'"; 
              }
              else {
                optiondata += "'"+resp.data[i].name+"',";
              }
            }
            console.log("onload:", optiondata); 
            resolve(optiondata); 
          } else {
            reject(this.status)
          }    
        };  
        ajaxRequest.send(null);
      });
    };

    request(queryString).then(function success(data) {
      console.log("returned data:", data)
    }, function error(err) {
      console.log(err)
    });

    var queryString = "?callFunction=get_all_data";
    var request = function(queryString) {
      return new Promise(function(resolve, reject) {
        var ajaxRequest = new XMLHttpRequest()
        , optiondata = "";
        ajaxRequest.open("GET", "https://example.com/data.txt", true);
        ajaxRequest.onload = function() {
          if (this.readyState === 4 && this.status === 200) {
            // do something
            optiondata = this.responseText;
            console.log("onload:", optiondata); 
            resolve(optiondata); 
          } else {
            reject(this.status)
          }    
        };  
        ajaxRequest.send(null);
      });
    };
    
    request(queryString).then(function success(data) {
      console.log("returned data:", data)
    }, function error(err) {
      console.log(err)
    });

Answer 4

Consider using the Promise object for asynchronous operations.

    var queryString = "?callFunction=get_all_data";
    var request = function(queryString) {
      return new Promise(function(resolve, reject) {
        var ajaxRequest = new XMLHttpRequest()
        , optiondata = "";
        ajaxRequest.open("GET", "php/shares.php" + queryString, true);
        ajaxRequest.onload = function() {
          if (this.readyState === 4 && this.status === 200) {
            // do something
            var resp = JSON.parse(this.responseText);
            console.log(resp);
            for (var i = 0; i < resp.data.length; i++) {
              if (i === resp.data.length -1) {
                optiondata += "'"+resp.data[i].name+"'"; 
              }
              else {
                optiondata += "'"+resp.data[i].name+"',";
              }
            }
            console.log("onload:", optiondata); 
            resolve(optiondata); 
          } else {
            reject(this.status)
          }    
        };  
        ajaxRequest.send(null);
      });
    };

    request(queryString).then(function success(data) {
      console.log("returned data:", data)
    }, function error(err) {
      console.log(err)
    });

    var queryString = "?callFunction=get_all_data";
    var request = function(queryString) {
      return new Promise(function(resolve, reject) {
        var ajaxRequest = new XMLHttpRequest()
        , optiondata = "";
        ajaxRequest.open("GET", "https://example.com/data.txt", true);
        ajaxRequest.onload = function() {
          if (this.readyState === 4 && this.status === 200) {
            // do something
            optiondata = this.responseText;
            console.log("onload:", optiondata); 
            resolve(optiondata); 
          } else {
            reject(this.status)
          }    
        };  
        ajaxRequest.send(null);
      });
    };
    
    request(queryString).then(function success(data) {
      console.log("returned data:", data)
    }, function error(err) {
      console.log(err)
    });

The XMLHttpRequest function successfully logs data in the console, but does not return the data within the function itself

Answer №1

Answer №2

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