How to make a "dot function" in javascript

Question

I'm trying to define a "dot function" where there are no parameters but has a . and a string or number before it like these:

.toUpperCase()
.toLowerCase()
.indexOf()
.charAt()
.substring()

You do 2..toString, not toString(2).

How do you define one of them?

This is not recommended. What if the official specification later defines a function with the same name but a different meaning? — John Dvorak
– John Dvorak, Commented Nov 9, 2013 at 6:00
Then you have to add any external methods to the prototype of objects — Voonic
– Voonic, Commented Nov 9, 2013 at 6:02
They'd have to be methods of an object and not everything behaves in a way that this would work for. — helion3
– helion3, Commented Nov 9, 2013 at 6:02
in any case, 2.toString() is invalid syntax. You would have to use 2..toString() — John Dvorak
– John Dvorak, Commented Nov 9, 2013 at 6:02

Paul Draper · Accepted Answer · 2013-11-09 06:07:19Z

31

Defining a "dot function" is easy. Here's how you can define it on a single object.

var a = {}, or a = function() {}, or a = [], etc.

a.dotFunction = function() { return 'hi'; }

console.log(a.dotFunction());

If you want to define it on all instances of a "class", use prototype.

function someClass() {
}

someClass.prototype.dotFunction = function() { return 'hi'; };

console.log(new someClass().dotFunction());

You can even do this on built-in types (some, like Prototype.js, do this, though most recommended against it).

Number.prototype.dotFunction = function() { return 'hi'; };

console.log((0).dotFunction());

answered Nov 9, 2013 at 6:07

Paul Draper

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Comments

p.s.w.g · Accepted Answer · 2013-11-09 06:07:41Z

19

I'd strongly recommend not trying to replace any built-in methods, however, you're free to define your own methods however you like.

You can do this by attaching the method to the Number or String type's prototype:

Number.prototype.foo = function(n) { return this * n; };
String.prototype.bar = function(n) { return this.length * n; };

alert(4..foo(2));  // 8
alert("4".bar(2)); // 2

Further Reading

Inheritance and the prototype chain

answered Nov 9, 2013 at 6:07

p.s.w.g

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2 Comments

jfriend00 Over a year ago

And you're not going to include any of the reasons why it's often bad to add methods to the built-in types? Also, why two periods in your first alert()?

alex Over a year ago

@jfriend00 I think JS parser needs 2 dots otherwise with only one lookahead it can't decide if its a fractional number or a dot for method access with only a single dot.

HMR · Accepted Answer · 2013-11-09 10:07:16Z

I'll give it a shot because nobody mentioned that you can already do this without having to define anything yourself.

A thing to take care of is if you have a number you have to place 2 dots after it where as if you have a function that returns a number or a variable that holds one you don't:

1..toString()
 .indexOf("1")//<=returns number 0
 //even though a number is returned we only need one dot here
 .toString();//<="0"

var num = 1234;
num.toString()//<=one dot
  .indexOf("23");//<=1

Your example would already work but since indexOf would return a number if you give it an argument that makes sense and a number doesn't have a charAt method.

"hello".toUpperCase()
  .toLowerCase()
  .indexOf("h")//<=returns a number 
               //number has no charAt method
  .toString()
  .charAt(0)
  .substring(0);//<="0"

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