# Immediately Invoked Function Expressions (IIFE)

**URL:** <https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860>\
**Category:** programming\
**Created:** [February 7, 2015, 6:20am UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860 "2015-02-07T06:20:39Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [February 7, 2015, 6:20am UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/1 "2015-02-07T06:20:40Z")

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This is a companion discussion topic for the original entry at [http://www.kirupa.com/html5/immediately\_invoked\_function\_expressions\_iife.htm](http://www.kirupa.com/html5/immediately_invoked_function_expressions_iife.htm)

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**Author:** ![krilnon](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/krilnon/32/34_2.png) [@krilnon](https://forum.kirupa.com/u/krilnon)\
**Post date:** [February 7, 2015, 7:29am UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/2 "2015-02-07T07:29:05Z")

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> [@kirupa](#):
>
> The main difference is that an IIFE leaves behind no evidence of its existence after it has run. This is largely because IIFEs are anonymous functions that are nameless.

This is true of the function itself, but there can be many traces (good and bad/accidental) that the function has run. The most accidental way is if you forget to declare a variable and use it. This will be detected as an error if you opt-in to ES5’s strict mode:

```javascript
(function(){ 'use strict'; foo = 5 })() // Uncaught ReferenceError: foo is not defined

```

If you aren’t using strict mode, which is the default, you will have just set the global variable `foo` in your program.

Plus, you can still mess with any global program state, even in strict mode, as long as those objects aren’t frozen:

```javascript
(function(){ 'use strict'; Math.PIE = 3 })()
Math.PIE // 3

```

So, as the tutorial mentions, IIFEs are great for protecting the IIFE code from the rest of your (or others’) programs, but they don’t stop your IIFE code from wreaking havoc on the code outside of your IIFE.

* * *

There are ways other than using wrapping parentheses to reach a valid point in the grammar so that your function is treated as an expression. Pretty much any unary operator will work, for example:

```javascript
void function(e){ console.log(e) }('hi')
!function(e){ console.log(e) }('hi')
typeof function(e){ console.log(e) }('hi')

```

I forget if there’s any good reason why people have settled on the paren syntax, other than that it reminds people of Lisp, I guess. 😛

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<div class="post-metadata">

**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [February 7, 2015, 7:38am UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/3 "2015-02-07T07:38:48Z")

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That’s a great point about IIFEs having the ability to wreck things outside of their bubbles. I’ll modify the tutorial by adding a note summarizing what you mention.

Regarding the syntax, the paren version does look at lot like Lisp haha!

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**Author:** ![senocular](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/senocular/32/7217_2.png) [@senocular](https://forum.kirupa.com/u/senocular)\
**Post date:** [February 8, 2015, 1:58pm UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/4 "2015-02-08T13:58:39Z")

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Void… Cooool. Looks like the return type 😜

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**Author:** ![richtone](https://avatars.discourse-cdn.com/v4/letter/r/bcef8e/32.png) [@richtone](https://forum.kirupa.com/u/richtone)\
**Post date:** [February 16, 2020, 9:50am UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/5 "2020-02-16T09:50:01Z")

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Hello, please make sure that what you write about is actually true and tested.

I have tried running this code in multiple enviroments (browser console, jscomplete-com, jsfiddle-net and playcode-io) :

```
function quotatious(names) {
    let quotes = [];
    
    for (let i = 0; i < names.length; i++) {   
        
        let theFunction = function() {
            return "My name is " + names[i] + "!";
        }
        
        quotes.push(theFunction);
    }
    return quotes;
}

// our list of names
let people = ["Tony Stark", "John Rambo", "James Bond", "Rick James"];

// getting an array of functions
let peopleFunctions = quotatious(people);

// get the first function
let person = peopleFunctions[0];

// execute the first function
alert(person());

```

And it runs **just fine** with “Tony Stark” being shown in the alert popup.

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**Author:** ![senocular](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/senocular/32/7217_2.png) [@senocular](https://forum.kirupa.com/u/senocular)\
**Post date:** [February 16, 2020, 4:42pm UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/6 "2020-02-16T16:42:21Z")

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@richtone This is a consequence of updating the site’s code samples to use more modern JavaScript, namely switching over from `var` to `let`. There is, however, a difference of behavior between these two types of declarations, one of which breaks this example. So by blinding making that replacement, this example no longer shows the broken behavior. If you change the `let`s to `var`s, you should see what was reportedly to be expected (specifically for the `for`).

@kirupa I want to think I remember this coming up for the book too, but I don’t remember what we did. Used `var` in this case? Removed this example?

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<div class="post-metadata">

**Author:** ![kirupa](https://yyz1.discourse-cdn.com/flex011/user_avatar/forum.kirupa.com/kirupa/32/11616_2.png) [@kirupa](https://forum.kirupa.com/u/kirupa)\
**Post date:** [February 16, 2020, 11:57pm UTC](https://forum.kirupa.com/t/immediately-invoked-function-expressions-iife/526860/7 "2020-02-16T23:57:52Z")

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Apologies @richtone. As @senocular mentions, this is an oversight from me doing a find/replace of `vars` into `lets`. I’m going to revise this tutorial to go back to using `var` with a note that none of this works when using `let`.

@senocular - the entire chapter was ✂