how to terminate parent function execution in javascript?

Question

I need to override some behaviour of function, that goes after it calls other function. The problem is that this parent function is a library and I dont want to change it, so solutions like make some flag or another change of this function is not so good. I know that I got an caller object in function that I can change, so maybe I can figure out smth with it. Heres the example:

function parent()
{
  console.log("some need stuff");
  some.handler.function.from.config.that.i.can.change();
  console.log("need omit this right till the end");
}

function child()
{
  console.log("need to somehow stop evaluation of " + child.caller + " function");
}

As a ruby programmer I know there is lambdas with which you can terminate evaluation from inner scope of closure. But Im not sure how to do this from javascript.

Answer 1

You can't do that directly. (Moreover .caller is obsolete)

You can however use a dirty trick:

try{
    parentFunction();//calls child
}catch(e){
   //done
}

function child(){
    doWhatever();
    throw new Error("this will hopefully propagate");
}

Fiddle

This will only work assuming the parent does not catch exceptions itself when calling the child.

Moreover, it is generally a bad idea to use exceptions for flow control. Use this as a last resort.

Answer 2

Call a new function that you do control before you call the library, and wrap the call to the library you cannot modify in a try/catch block.

For example:

function meta-parent()
{
  try {
    parent();
  }
  catch (e){
      // don't really be empty!
  }
}

function parent()
{
  console.log("some need stuff");
  some.handler.function.from.config.that.i.can.change();
  // Nothing below here will run because of your child's exception
  console.log("need omit this right till the end");  
}

function child()
{
  console.log("need to somehow stop evaluation of " + child.caller + " function");
  throw new Error(); //breakout!
}

Note inspiration: Breaking a parent function from within a child function (PHP Preferrably)

Answer 3

As Benjamin Gruenbaum points out, the use of exceptions for flow control should be regarded as a last resort, however does the Promises/A proposal (in some implementations) not legitimize such exploitation of exceptions in this way?

I do take on board that an (otherwise uncaught) error thrown from a child function is maybe not what the Promises/A authors had in mind (ie we're still arguably talking about a "dirty trick"), however the Promises/A proposal doesn't specifically exclude such error propagation either.

The Q lib would be ideal for this problem, allowing something like this :

Q.fcall(parentFunction).then(function (value) {
    alert(value);
}, function (error) {
    alert(error);
});

fiddle

Please note that, for convenience, I also use jQuery in the fiddle but only to give functionality to the buttons.