How to avoid loosing 'this' in the callbacks?

Question

Object on which a function is called gets lost subsequently in the callbacks. How could I preserve/maintain the object on which the function was called.

var javaci = new Javaci();
javaci.run(actionObject.source);
javaci.once('f2run',this.output);
javaci.once('jvmo',this.output);

The above snippet (which is altogether in a different class)calls a run method of class Javaci and then listens for 2 events, f2run and jvmo.

But the callback function this.output is never called.

The following function successfully calls the subsequent compileAndRun function.

Javaci.prototype.run = function(code) {
    if(code) {
      var classDetails = this.classToInject(code);
      if(Object.keys(classDetails).length > 0) {
        code = `${classDetails['classToInject']}${code}`;
        var writer = new Writer();
        writer.write(code,`${classDetails['className']}.java`);
        writer.once('written',this.compileAndRun);
        writer.once('failed',this.failedToWrite);   
      }
    }
}

When the control enters the following function, this refers to the object of Writer class. How could I avoid this? Perhaps, this is the reason, the event listener function is never fired.

Javaci.prototype.compileAndRun = function(fileName) {
    if(fileName) {
    const child2 = execFile('javac', [fileName], (error, stdout, stderr) => {
      if (error) {
        this.emit('f2run',error);
      }else {
          var classFile = fileName.replace('.java','').trim();
          execFile('java', [classFile], (error, stdout, stderr) => {
          if (error) {
            this.emit('f2run',error);
          }else {
            this.emit('jvmo',stdout);
          }
        });
      }
    });
    }
}

@KevinB That was all! I knew of arrow functions, but thought they had to be defined inline. — Suhail Gupta, Dec 05 '16 at 16:21

score -1 · Answer 1 · answered Dec 05 '16 at 16:01

this usually refers to global in callbacks,you can try this writer.once('written',this.compileAndRun()); /*******************************************/ Javaci.prototype.compileAndRun = function () { var self=this; return function(fileName) { if(fileName) { const child2 = execFile('javac', [fileName], (error, stdout, stderr) => { if (error) { self.emit('f2run',error); }else { var classFile = fileName.replace('.java','').trim(); execFile('java', [classFile], (error, stdout, stderr) => { if (error) { self.emit('f2run',error); }else { self.emit('jvmo',stdout); } }); } }); } } }

How to avoid loosing 'this' in the callbacks?

1 Answers1