AJAX, pass additional variable to callback and store XMLHTTLRequest.response to variable

Question

I am trying to read a local file on the server with a standard function loadDoc(url, cfunc), then

1) search for a particular string in the file (getLine());

2) if possible, store that line to a variable.

For point 1 I pass a string to the callback. 2) Getting the response is problematic because XMLHTTPRequest is asynchronous. At this moment the error is: "ReferenceError: xhttp is not defined"

function main(){
    var url="data.txt"
    var str="1,0,"; //just an example
    var myCallBackWithVar = function(){
        getLine(str);
    };
    loadDoc(url, myCallBackWithVar);

    //Can I get the line here somehow?
}

function loadDoc(url, cfunc) {
    var xhttp=new XMLHttpRequest();
    xhttp.onreadystatechange = function() {
        if (xhttp.readyState == 4 && xhttp.status == 200) {
        cfunc(xhttp);
        }
    }
    xhttp.overrideMimeType('text/plain');
    xhttp.open("GET", url, true);
    xhttp.send();
}

//Find string with the desired data in txt file
function getLine(str) {
    var data=xhttp.responseText;
    //Find the line from the txt file
    var start=data.indexOf(str);
    var end=data.indexOf(";",start);
    var line=data.substring(start,end);    
    return line;
}

data.txt is something like this:

some data here
0,0,9;
1,0,10;
1,1,11;

I have already tried to pass the XMLHTTPRequest objetct getLine(xhttp,str). How to solve points 1 and 2? I'd rather keep it jQuery free for the moment. Thanks

Ruben Yeghikyan · Accepted Answer · 2015-10-10 20:33:29Z

1

Can I get the line here somehow?

I don't think that's a good idea. You can't be sure that your app will work correctly. XHR is a async function and you should use async architecture.

Here the example how this functionality can be done.

    var text; // define global variable
    var str = "1,0,"; //just an example

    function main(){
        var url = "data.txt";
        var cb = function (data){
            text = getLine(data);
            // you can use text var here
            // or in anyewhere in your code
        }

        loadDoc(url, cb);
    }

    function loadDoc(url, cb) {
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function() {
            if (xhr.readyState == 4 && xhr.status == 200) {
                cb(xhr.responseText);
            }
        }
        xhr.overrideMimeType('text/plain');
        xhr.open("GET", url, true);
        xhr.send();
    }

    //Find string with the desired data in txt file
    function getLine(data) {
        if(data) {
            //Find the line from the txt file
            var start = data.indexOf(str);
            var end = data.indexOf(";", start);
            var line = data.substring(start, end);

            return line;
        }
    }

edited Oct 10, 2015 at 20:33

answered Oct 10, 2015 at 18:12

Ruben Yeghikyan

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2 Comments

aless80 Over a year ago

In getLine I get str is not defined even if I add the definition of str in main. how to pass that as well?

Ruben Yeghikyan Over a year ago

Updated. If you define a variable inside a function that var will be visible only in that function. To define global variable you need to use window object like this window.myVar = 'something'; Or just define variable in global scope (not inside a function).

David Li · Accepted Answer · 2015-10-10 18:09:42Z

1

On complete, you don't need to pass the whole xhttp variable through too the callback function. When you do this:

function getLine(str) {
    var data=xhttp.responseText;

xhttp is already out of scope. To fix this, the parameter name would also have to be xhttp.

A better way would be to do :

cfunc(xhttp.responseText);

and then

var data=str

This way, you are passing only what you need as an argument.

answered Oct 10, 2015 at 18:09

David Li

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