Why does this javascript code using prototype work?

Question

I have this javascript code below:

<!doctype html>
<html lang="en">
    <head>
        <meta charset="utf-8" />
        <title></title>

        <script>
            function Person(first, last) {
                this.first = first;
                this.last  = last;
            }

           Person.prototype.toString = function() { 
                return this.first + this.last;
           }
            var person = new Person("John", "Dough");

            alert(person); // same result since alert calls toString()  

        </script>
    </head>
    <body>
    </body>
</html>

The question is why does the alert(person) display "JohnDough"? To me, alert(person) should not display anything.

alert calls toString() of the passed object, just like the comment says: alert calls toString(). And toString is overwritten on the prototype to concatenate the first and last name. — Felix Kling
– Felix Kling, Commented Jan 15, 2013 at 15:06

Ian · Accepted Answer · 2013-01-15 15:06:39Z

5

When using alert, the method implicitly attempts to call a toString on the object. In your case, toString is defined and does what you expect when explicitly calling toString. If you hadn't defined toString, alert would have used the native toString method of an Object and returned "[object Object]", as pointed out by @FelixKling.

answered Jan 15, 2013 at 15:06

Ian

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4 Comments

Felix Kling Over a year ago

The default string representation of an object is [object Object]. That's what you would see. Example: jsfiddle.net/BN8L7.

Ian Over a year ago

@FelixKling Yes yes, thank you. Don't know why I didn't think of that, or tried it in a fiddle or something

jim dif Over a year ago

Ok Ian I think I get it. if I remove the toString method then the alert function alerts object Object . If I change the method to toString1 the alert uses object Object again. Only because toString was defined the alert replied JohnDough and only for that reason ?

Ian Over a year ago

@jimdif Yeah, simply because you defined it, it basically "overrides" the native implementation of returning "[object Object]"

Ashley Sheridan · Accepted Answer · 2013-01-15 15:07:02Z

1

It's because that object has a toString() method. alert() requires a string, and will use this method for an object if it exists, or its own built-in one if not. As the method here returns the forename and surname, that's what you get in the alert() dialogue.

answered Jan 15, 2013 at 15:07

Ashley Sheridan

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1 Comment

jim dif Over a year ago

Ashley I am not sure if what you are saying is correct. The name JohnDough will not be alerted if the toString method does not exist via the prototype declaration. If it does not then object Object is alerted and not JohnDough .

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