javascriptjquerysplitpausing-execution

Wiered behaviour by Javascript


I have the following Java Script code fragment:

function upoload_TC(evt) {
    var file = evt.target.files[0];
    if(file.type != 'text/plain'){
        evt.target.style='color:red;';
    }
    else{
        var app_data='';
        reader = new FileReader();
        reader.onload = function(){
            app_data = reader.result;
        };
        reader.readAsText(file);

        if (evt.target.id[7]=='2') {
/* area of intrest begin: */
            var packet_order = get_packet_order(evt.target);
            console.log("1st");
            var app_data1 = app_data.split('\n');
            console.log("app_data: ");
            console.log(app_data);
            console.log("app_data1: ");
            console.log(app_data1);
/* area of intrest end */
            for(var i=0; i<app_data1.length; ++i){
                console.log("3st");
                if(app_data1[i][0] == '!'){
                    app_data1.splice(i,1);
                    --i;
                    console.log(app_data1);
                }
                console.log(app_data);
                ...
            }
        }
    }
}

app_data have a long string.

After execution, sometimes app_data1 do not have any content logged.
Observation: When I execute it step by step in the debugger, app_data1 has the expected value. But if my 1st break point is after the assignment statement for app_data1, it's empty.

How can I fix this?

I found a call_back and promises to tackle this kind issues for user-defined functions. Since split() is not defined by me, I think these won't work.
I believe timeout is not the correct way to do this. Is it?

Please find the video of debugger window here.


Solution

  • You should be doing the logging / processing inside the onload callback. In your example code, you're setting the value for app_data on the load event, which is fired after the asynchronous function readAsText finishes its thing. By that time the logging / processing code has already been executed.