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.

What is the purpose of creating an alias for `.length`? Note, `this` with IIFE is not `String` — guest271314, Sep 10 '16 at 23:10
is this kind of homework? There was already a suspiciously similar question from half an hour ago: http://stackoverflow.com/questions/39431230/add-an-alias-for-a-property-in-javascript — VLAZ, Sep 10 '16 at 23:11
Possible duplicate of [Javascript getters and setters for dummies?](http://stackoverflow.com/questions/812961/javascript-getters-and-setters-for-dummies) — Siguza, Sep 10 '16 at 23:13
that's why I'm asking, because I answered on the question you mentioned, but I realised my answer wasn't correct. @Siguza you read my note I hope — Lyubomir, Sep 10 '16 at 23:13

score 2 · Accepted Answer · answered Sep 10 '16 at 23:17

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.

Ruan Mendes · Answer 2 · 2016-09-10T23:24:33.003

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())

JavaScript - prototype as a self invoking function (IIFE)

2 Answers2