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1

I have three questions and all of them are related.

1) I want to add a property called bar to String object. I did this

String.prototype.bar=function() {console.log("jjjj");}
"mystring.bar//function()
"mystring".bar()//jjjj

but I want to use "mystring".bar (no function call) just like how we use "mystring".length to get the length of string. how can I acheive this?

2) now I want to change the inbuilt method of length on String object. so I did this

>>> String.prototype.length=function(){console.log("xxx");};
function()
>>> "mmmm".length
4

but the length method has not changed. it still returns the length of string only. Note: I am changing this method only on console for learning purposes

3) I am having difficulty understanding this function from the book, javascript, the good parts by Crockford.(pg no 40) here is the method to augment String object. It replace HTML entities in a string and replace them with their equivalents

String.method('deentityify',function() {
var entity =  {
quot:'";,
lt:'<',
gt:'<'
};
//return the deentityify method
return function() {
  return this.replace(/&([^&;]+);/g,
         function (a,b) {
         var r = entity[b];
         return typeof r==='string' ? r : a;
        }
     );
   };
}());
'&lt;&quot;&gt;'.dentityify();//<">

questions on this problem: 1)since no prototpye is used, does this method is available on all String objects to be used.

2)in the return this.replace(.. part of the above, I do not understand what parameters are given to a and b. ie when I call '&lt;&quot;&gt;'.dentityify();, what does a and b get and how that anonymous function is executed.

Thank you all for the help

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2 Answers 2

up vote 2 down vote accepted

I want to use "mystring".bar (no function call) just like how we use "mystring".length to get the length of string. how can I acheive this?

Assuming you want to use a function to compute some value dynamically, but want to use the syntax of a normal property access, not a function, you have to create a getter property with Object.defineProperty:

Object.defineProperty(String.prototype, 'bar', {
    get: function() {
        return 'jjjjj';
    }
});

but the length method has not changed. it still returns the length of string only.

That's because .length is not a writeable property:

Once a String object is created, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

I do not understand what parameters are given to a and b [...] and how that anonymous function is executed

The callback is executed internally by .replace for each match found by the regular expression. What the arguments are is explained in the MDN documentation. The first argument is the whole match and each following argument is the value of one of the capture groups in the expression.

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up vote 2 down vote

Regarding (1) - don't use function, assign property directly to prototype:

String.prototype.bar = "bar from string prototype";
alert('some text'.bar); // "bar from string prototype"

Regarding (2) - I don't think this is possible.

Regarding (3) Crockford defines .method as:

Function.prototype.method = function(name, func) {
   this.prototype[name] = func;
   return this;
};

So it's a custom implementation designed to hide using of the prototype

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