Change an existing js method without changing the original js

Question

I have a class that gets created (testClass for this purpose). The class has the following method inside of it:

setExternalValue: function(val) {
        this.sliderCaption.innerHTML =  getPointsString(val*this.points_per_use, this.points_currency);
        this.sliderValuebox.value = this.getUses();
    }

How can I tie into this and listen for this method to happen? When it's completed, I would like to run a specific function. Putting a custom event inside of the setExternalValue will not work for me because I cannot edit the original js.

can't you just alert(setExternalValue(value)) ? It doesn't seem like setExternalValue is doing any async process. — DinoMyte
– DinoMyte, Commented Oct 22, 2015 at 18:12
Once this method happens I need to perform an ajax call to update some data within the page. — thindery
– thindery, Commented Oct 22, 2015 at 18:14

Paul Roub · Accepted Answer · 2015-10-22 18:21:53Z

3

You can wrap it, call the original function, then insert your handler code.

var myObj = new testClass();   // or however you normally initialize them

myObj.oldSetExternalValue = myObj.setExternalValue;

myObj.setExternalValue = function(val) {
   var result = this.oldSetExternalValue(val);

   // whatever you wanted to run after it completes

   return result;
}

var testClass = function() {
  this.setExternalValue = function(val) {
    console.log("original setExternalVal " + val);
    return "return value";
  }
}

var myObj = new testClass();

myObj.oldSetExternalValue = myObj.setExternalValue;

myObj.setExternalValue = function(val) {
  var result = this.oldSetExternalValue(val);

  console.log("wrapper called with " + val + " after " + result);

  return result;
}

myObj.setExternalValue("foo");

edited Oct 22, 2015 at 18:21

answered Oct 22, 2015 at 18:14

Paul Roub

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1 Comment

thindery Over a year ago

Thank you. I got this working. I knew it was something along these lines.

Will N · Accepted Answer · 2015-10-22 18:15:02Z

1

Make a reference to the old function. Replace the old function with something that calls the old function but also performs new stuff when it finishes.

var obj = {   
    setExternalValue: function(val) {
        alert("inside method: " + val);
    }
};
var referenceOldFunction = obj.setExternalValue;

obj.setExternalValue = function (val) {

    referenceOldFunction.call(obj, val);

    // do stuff after it ends
}

obj.setExternalValue("test");

answered Oct 22, 2015 at 18:15

Will N

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1 Comment

Will N Over a year ago

Paul Roubs is better than mine.

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