What is the purpose of var foo = (function() {

Question

I came across following code. Can someone explain it to me? I'm not sure how it'll be executed? Is there any specific reason to write it like this?

var foo = (function() {
  // ...bunch of code
  return function(param1) {
    // ...bunch of code
    return var1;
  }
})();

EDIT: I know that the outer part (...)();, is executed immediately. What I'm not sure is why it is assigned to variable foo. I guess when the IIFE is executed it'll only execute the return ... inner code. But when the code inside that inner function ... is executed? When the line return var1 is executed? I hope now it is clear as water

Possible duplicate of [What is the (function() { } )() construct in JavaScript?](http://stackoverflow.com/questions/8228281/what-is-the-function-construct-in-javascript) — George Kagan, Nov 06 '16 at 20:46
Possible duplicate of [A self invoking anonymous function expression](http://stackoverflow.com/questions/18203897/a-self-invoking-anonymous-function-expression) — technophobia, Nov 06 '16 at 20:47
It is a IIFE. It could be executed a a later time. Also, it helps to avoid repeatedly having to create an object that will have the same behavior. — Gacci, Nov 06 '16 at 20:50
@gmailuser I've updated my answer, I hope things get clearer — acontell, Nov 06 '16 at 21:04

acontell · Accepted Answer · 2016-11-06T21:32:49.457

What you're seeing is a JS idiom called Immediately-invoked function expression. Extracted from Wikipedia:

An immediately-invoked function expression (or IIFE, pronounced "iffy"1) is a JavaScript programming language idiom which produces a lexical scope using JavaScript's function scoping. Immediately-invoked function expressions can be used to avoid variable hoisting from within blocks, protect against polluting the global environment and simultaneously allow public access to methods while retaining privacy for variables defined within the function. This concept has been referred to as a self-executing anonymous function,[2] but Ben Alman introduced the term IIFE as a more semantically accurate term for the idiom, shortly after its discussion arose on comp.lang.javascript

As can be read in the extract, it's main purpose is mainly to avoid polluting the global namespace though it is also heavily used to take advantage of closure.

UPDATE

In Javascript, functions are first-class objects. This means that they can be passed around like any other object would be. They can also be assigned to variables.

The function you mention is just returning a function (like it could be returning an int or an object literal) that gets assigned to the variable foo. foo then has the reference to the function and can be executed like any normal function would be (i.e foo('a') ).

By the way, a function that returns another function or takes one or more functions as arguments is called higher-order function.

Hope this clarifies things.

Also it is a function factory in this case. Defining and returning a function with some hardcoded variables from the outer scope. — Ali Naci Erdem, Nov 06 '16 at 20:50
Yes, it takes advantage of closure, it is a well-known/used pattern — acontell, Nov 06 '16 at 20:55

score 0 · Answer 2 · answered Nov 06 '16 at 20:50

The reason would be difficult to guess, but it could be for obfuscation, to avoid plagiarism

The execution will go like:

var foo will receive the result of a function call (the parenthesis after the assignment contain a function declaration, and right after those, the other pair of parenthesis are those function arguments)
that 1st anonymous function will execute the bunch of code, then return what the other function evaluates
the 2nd anonymous function well execute the other bunch of code and finally return var1
ergo: it is the same of executing bunches of code, store the result in var1 and then var foo = var1, simplifying for explanation purpose

What is the purpose of var foo = (function() {

2 Answers2