A question about JavaScript's TypeError exception

StackOverflow https://stackoverflow.com/questions/4866940

Question

I've 3 questions. Thank you!

First question:

When will JavaScript codes cause a "TypeError" exception?

Other questions:

I've codes below:

<!DOCTYPE html>
<meta charset="utf-8">
<title>An HTML5 document</title>
<script>
    var str = 'abc'; // str's type is string, not object

    // Syntax: Object.getPrototypeOf(object)
    alert(Object.getPrototypeOf(str)); // Uncaught TypeError: Object.getPrototypeOf called on non-object

    // Syntax: prototype.isPrototypeOf(object)
    if (Object.prototype.isPrototypeOf(str)) { // false
        alert('true');
    } else {
        alert('false');
    }
</script>

Method getPrototypeOf() and isPrototypeOf() are both need a parameter which type should be object. And str's type is string.

Why getPrototypeOf method throws an TypeError exception, and isPrototypeOf method doesn't throws any errors?

If str's type is object (var str = new String('abc')), the result of Object.prototype.isPrototypeOf(str) is true. But the result of codes above is false. Why isn't str converted from string to object automatically when it's used as a parameter of isPrototypeOf method?

Thank you!

Was it helpful?

Solution

  1. Take a look at the first hit for "TypeError mdc". When it throws a typeerror is upto the specification and the user.

The other answeres the specific question.

OTHER TIPS

My theory is that isPrototypeOf is kind of like the brother to the instanceof operator so they really should have the same basic semantics. Also new functions in ECMAScript 5 tend to be a bit more strict when compared to functions in older editions. Here are the algorithms used.

15.2.3.2  Object.getPrototypeOf ( O ) 

When the getPrototypeOf function is called with argument O, 
the following steps are taken: 

1.  If Type(O) is not Object throw a TypeError exception. 
2.  Return the value of the [[Prototype]] internal property of O. 

15.2.4.6  Object.prototype.isPrototypeOf (V) 

When the isPrototypeOf method is called with argument V, 
the following steps are taken: 

1.  If V is not an object, return false. 
2.  Let O be the result of calling ToObject passing the this value as 
    the argument.  
3.  Repeat 
  a.  Let V be the value of the [[Prototype]] internal property of V. 
  b.  if V is null, return false 
  c.  If O and V refer to the same object, return true.
Licensed under: CC-BY-SA with attribution
Not affiliated with StackOverflow
scroll top