为什么“String.prototype = {}”不起作用？

Question

I wrote this code in javascript:

String.prototype = {
  a : function() {
    alert('a');
  }
};

var s = "s";
s.a();

I expect it alert an a , but it reports:

s.a is not a function

Why?

Answer 1

You seem to be replacing the entire prototype object for String with your object. I doubt that will even work, let alone be your intention.

The prototype property is not writable, so assignments to that property silently fail ( @Frédéric Hamidi ).

Using the regular syntax works, though:

String.prototype.a = function() {
  alert('a');
};

var s = "s";
s.a();

Answer 2

you have to write like :

String.prototype.a = function(){
alert("a");
};

var s = "s";
s.a();

fiddle : http://jsfiddle.net/PNLxb/