JavaScript - prototype as a self invoking function (IIFE)

Question

I would like to define a prototype on String that is a self-invoking function

String.prototype.overEstimatedLength = (function() {
   return this.length + 12345
})()

and then use it like this

'hello world'.overEstimatedLength

Unfortunately, this doesn't work. Is something like that syntactically possible and why is the example above not working?

Note: I know that property definition would be more appropriate (such as a Getter), I'm specifically interested in self-invoking functions.

Answer 1

The problem with your example is that there isn't actually such a thing as a "self-invoking function", only an " immediately -invoked function expression", with the emphasis on immediately .

Consider something like this:

String.prototype.foo = alert('foo');
'foo'.foo;
'foo'.foo;

This runs alert('foo') immediately , then stores the result in String.prototype.foo . . . and then just retrieves that result a few times (doing nothing with it). So 'foo' will get alerted only once.

Your example is similar; you are immediately invoking your function expression.

Answer 2

It seems like you are trying to define a getter on String.prototype

 Object.defineProperty(String.prototype, 'overEstimatedLength', { get: function() { return this.length + 12345; } }); console.log('hello'.overEstimatedLength) 

Your code doesn't work because it's executing your function immediately and assigning the result of that to String.prototype.overEstimatedLength . That is, it's almost exactly the same as...

function myFunc() {
    return this.length + 12345
}
String.prototype.overEstimatedLength = myFunc();

The following would kind of work, but you'd call it as a function, notice that you are returning a function, so that gets assigned to String.prototype

 String.prototype.overEstimatedLength = (function() { return function() { return this.length + 12345; } })() console.log('something'.overEstimatedLength())