Cannot read property 'apply ' of undefined

Question

I am trying to understand the .apply-method. So, I created the following code:

var o = new Object();
o.method = function(x, y) {
  return x + y
};

o.method = (function(original) {
  return function(x, y) {
    var a = 'A';
    var b = 'B';
    var result = original.apply(this, [a, b]);
    return result;
  }
})(o.method);

console.log(o.method(1, 2));

I thought that the last open and close parentheses could be empty and so the method o.method is invoked automatically. But with empty parentheses

o.method = (function(original) {
  return function(x, y) {
    var a = 'A';
    var b = 'B';
    var result = original.apply(this, [a, b]);
    return result;
  }
})();

o.method(1, 2);

I get the following error: Cannot read property 'apply' of undefined

Can someone please explain me why?

Kind regards Henning

Possible duplicate of [Detecting an undefined object property](https://stackoverflow.com/questions/27509/detecting-an-undefined-object-property) — Poorna Senani Gamage, Oct 11 '18 at 19:33

score 1 · Answer 1 · answered Oct 11 '18 at 19:43

If you invoke the function function(original) {} you return a the function function(x, y) {} and you create a closure over the parameter original. So the code inside of the function(x, y) {} has allways access to what ever you passed to function(original) {}

With:

o.method = (function(original) {
  return function(x, y) {
    // ... 
  }
})(o.method);

You assign that returned function(x, y) {} function having a closure over original to o.method passing the original o.method as argument.

If you now write o.method(1,2) you call that returned function(x, y) {}.

If you called that function(original) {} with no argument (function(original) {})() then original is undefined and original.apply would result in your posted error.

Thank your for your answer. So ... with the last both parantheses I define the argument for the parameter original? — readme_txt, Oct 11 '18 at 20:00

Behrouz Pooladrak · Answer 2 · 2018-10-11T20:12:25.373

With some change your code is correct:

var o = new Object();
o.method2 = function(x,y){return x+y};

o.method = (function (original){
    return function(x,y){
            var result = original.call(this, x,y);
        return result;
    }
})(o.method2);

Also for test and run:

o.method(2,3)

Result is 5

Also with Apply:

var o = new Object();
o.method2 = function(x,y){return x+y};

o.method = (function (original){
    return function(x,y){
            var result = original.apply(this, [x,y]);
        return result;
    }
})(o.method2);

Then test:

o.method(2,3)

Result is 5

For more information about the usage of call() and apply() you can read this article: http://adripofjavascript.com/blog/drips/invoking-javascript-functions-with-call-and-apply.html

Cannot read property 'apply ' of undefined

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