I got stumped by this weirdness.

Let's say I have this array:

var array = [{
  something: 'special'
}, 'and', 'a', 'bunch', 'of', 'parameters'];

Could I apply the apply method of a function to invoke the function with the this object being {something: 'special'} and the parameters being the rest of array?

In other words, could I do this

var tester = function() {
  console.log('this,', this);
  console.log('args,', arguments);
};
tester.apply.apply(tester, array);

And expect the output to be the following?

> this, {"something": "special"}
> args, {"0": "and", "1": "a", "2": "bunch", "3": "of", "4": "parameters"}

I tried it.

TypeError: Function.prototype.apply: Arguments list has wrong type

But why? It seems like this should work.

I got stumped by this weirdness.

Let's say I have this array:

var array = [{
  something: 'special'
}, 'and', 'a', 'bunch', 'of', 'parameters'];

Could I apply the apply method of a function to invoke the function with the this object being {something: 'special'} and the parameters being the rest of array?

In other words, could I do this

var tester = function() {
  console.log('this,', this);
  console.log('args,', arguments);
};
tester.apply.apply(tester, array);

And expect the output to be the following?

> this, {"something": "special"}
> args, {"0": "and", "1": "a", "2": "bunch", "3": "of", "4": "parameters"}

I tried it.

TypeError: Function.prototype.apply: Arguments list has wrong type

But why? It seems like this should work.

• javascript

• tester.apply(tester, array); is what you want... – dandavis Commented Jul 16, 2013 at 20:46
• 2 @dandavis Not really. The value of this should be array[0], and the arguments passed should be array[1] to array[len-1] – Ian Commented Jul 16, 2013 at 20:47
• @Ian: sorry, misunderstood. eval.call.apply(tester, array) works, since you want to apply the call (confused yet, heh).. – dandavis Commented Jul 16, 2013 at 20:49

2 Answers 2

But why?

Let's reduce your calls step-by-step:

tester.apply.apply(tester, array) // resolves to
(Function.prototype.apply).apply(tester, array) // does a
tester.apply({something: 'special'}, 'and', 'a', 'bunch', 'of', 'parameters');

Here you can see what's going wrong. Correct would be

var array = [
    {something: 'special'},
    ['and', 'a', 'bunch', 'of', 'parameters']
];

then the apply.apply(tester, array) would bee

tester.apply({something: 'special'}, ['and', 'a', 'bunch', 'of', 'parameters']);

which does a

tester.call({something: 'special'}, 'and', 'a', 'bunch', 'of', 'parameters');

So with your original array you'd need to use

(Function.prototype.call).apply(tester, array)

The apply method takes one argument for the this context and one argument for the arguments, that you want to apply. The second argument must be an array.

tester.apply.apply(tester, array);

Due to the second apply method, the first will be called like this:

tester.apply({something: 'special'}, 'and', 'a', 'bunch', 'of', 'parameters');

And since 'and' is not an array, you get the TypeError that you described. You can easily fix this by using the call method:

tester.call.apply(tester, array);

call will take individual arguments instead of an array, which will result in the desired oute.