Why is a method on an object being true?

Question

Can you explain me what's the difference between ModelState.IsValid and ModelState.IsValid() in the example below? (Both of them are working)

var func = function () {
  var ModelState = {
    IsValid: function () {
      return true
    }
  };
  
  // ModelState.Isvalid() is working, too
  document.body.innerHTML = ModelState.IsValid 
      ? 'ModelState is valid' : 'ModelState is invalid'
};

<button onclick="func()">Click me</button>

As you can see above, ModelState is a json object. If I can call ModelState.IsValid, is IsValid called json property in this case?

ModelState is not a JSON object. Its an object literal. And IsValid is a property of the ModelState object literal. — Tarun Dugar, Jan 23 '16 at 07:35
@TarunDugar is correct. I think you are confused on the the difference between calling a function without parenthesis. Here is a similar question to help you http://stackoverflow.com/questions/3246928/in-javascript-does-it-make-a-difference-if-i-call-a-function-with-parentheses — John Roca, Jan 23 '16 at 07:42

gurvinder372 · Answer 1 · 2016-01-23T07:45:16.017

1

If I can call ModelState.IsValid, is IsValid called json property in this case?

ModelState is an object literal and isValid is a property of this object.

if you change your code to

document.body.innerHTML = ModelState.Isvalid 
      ? 'ModelState is valid' : 'ModelState is invalid'

it will output ModelState is invalid

edited Jan 23 '16 at 07:45

answered Jan 23 '16 at 07:38

gurvinder372

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you've changed nothing with the code – Tân Jan 23 '16 at 07:44
Why? `ModelState.IsValid` is a non-null method, which is truthy, so it seems this should print `modelState is valid`. – Jan 23 '16 at 07:44
@torazaburo sorry, someone edited the code and for some reason that edit was accepted!! – gurvinder372 Jan 23 '16 at 07:45

Rajesh · Accepted Answer · 2016-01-23T07:43:38.780

1

IsValid is a function. So when you do this ModelState.IsValid, this will return the function and not its value.

ModelState.IsValid() this will execute it and give you return value.

var func = function () {
  var ModelState = {
    IsValid: function () {
      return true
    }
  };
  
  console.log("ModelState.IsValid: ", ModelState.IsValid)
  
  console.log("ModelState.IsValid():", ModelState.IsValid())
  
  // ModelState.Isvalid() is working, too
  document.body.innerHTML = ModelState.IsValid 
      ? 'ModelState is valid' : 'ModelState is invalid'
};

<button onclick="func()">Click me</button>

Now, why ternary operator works:

JS tries to convert value to Boolean. If this is not possible, then availability is checked. So when you do

if("")

it will convert "" to 0 and 0 is considered as false, but if it has value, it is considered as true.

var a = "";

alert(a?true:false);

a = "function(){...}";

alert(a?true:false);

edited Jan 23 '16 at 07:43

answered Jan 23 '16 at 07:39

Rajesh

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Thanks! I've tried and seen the difference. But when I change `document.body.innerHTML = ModelState.IsValid ? 'ModelState is valid' : 'ModelState is invalid'` to `document.body.innerHTML = ModelState.IsValid() ? 'ModelState is valid' : 'ModelState is invalid'`. I'm getting same result. – Tân Jan 23 '16 at 07:41
Can you tell me more about that? – Tân Jan 23 '16 at 07:42
I have updated my answer. Hope that helps – Rajesh Jan 23 '16 at 07:44
it looks like `implicit conversion 'string' to 'boolean'` – Tân Jan 23 '16 at 07:51
1

Yes. You can refer [following post](http://stackoverflow.com/questions/8692982/in-javascript-is-an-empty-string-always-false-as-a-boolean) for reference. – Rajesh Jan 23 '16 at 07:53
Many thanks! That's really help :) – Tân Jan 23 '16 at 07:57

Why is a method on an object being true?

2 Answers2