how can i add custom properties to javascript string object

Question

i want to define my own custom properties for string object .so that i can use this properties directly on string objects. eg:- str is my string object.then i should be able to use .IsNull property as shown below.

var str = “string”;

str.IsNull; //returns true if null

str.IsEmpty; returns true if empty

A string can't be null (strings and null are different types). — nnnnnn
– nnnnnn, Commented Sep 12, 2016 at 7:17
C# allows null string.thanks didn't know this that js don't have null strings — Pawan Wagh
– Pawan Wagh, Commented Sep 12, 2016 at 9:50

brk · Accepted Answer · 2016-09-12 07:08:47Z

3

You can explore prototype property

For example

String.prototype.yourMethod = function(){
 // Your code here
}

answered Sep 12, 2016 at 7:08

brk

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4 Comments

Randy Over a year ago

Please note that you can't check if a string is null with a method in the String prototype, because the type would be null and not String.

nnnnnn Over a year ago

But your example adds a method; the OP is trying to add properties (or perhaps getter methods that seem like properties to the client code).

Pawan Wagh Over a year ago

why can't string be null in js?

Randy Over a year ago

@PawanWagh Because null is a type just like string. It cannot be of both types at the same time. It's eigher a string, maybe an empty one, or it is null (or undefined, which is something else too)

Pawan Wagh · Accepted Answer · 2016-09-12 09:57:16Z

3

thanks to all of you for your help.but i got the way

Object.defineProperty( String.prototype, 'IsNullOrEmpty', {
    get: function () {
        return ((0 === this.length) || (!this) || (this === '') || (this === null));
    }
});

var str = "";
str.IsNullOrEmpty;//returns true

answered Sep 12, 2016 at 9:57

Pawan Wagh

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Comments

EternalLight · Accepted Answer · 2016-09-12 07:14:30Z

1

You have to add a new method to a String wrapper object's prototype. The best practice is to check if the method already exists before declaring it. For instance:

String.prototype.yourMethod = String.prototype.yourMethod || function() {
    // The body of the method goes here.
}

answered Sep 12, 2016 at 7:14

EternalLight

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1 Comment

nnnnnn Over a year ago

I don't see how "checking" if the method exists helps if you then just proceed blindly on the assumption that the pre-existing method will work exactly the same way as the one you're trying to add. (Should be fine if polyfilling standard JS methods for older browsers, but could go horribly wrong for custom methods.)

Randy · Accepted Answer · 2016-09-12 07:17:11Z

1

I would personally make functions for this instead of extending the prototype.

If you extend the prototype, you have to make sure you add them to all types you want to check, it goes beyond the String object.

function isNull(str) {
  console.log( str === null );
}

function isEmpty(str) {
  console.log( typeof str == 'string' && str === '' );
}


isNull(null);
isNull('');
isNull('test');

isEmpty(null);
isEmpty('');
isEmpty('test');

answered Sep 12, 2016 at 7:17

Randy

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