What is this called in a Javascript Variable function?

Question

What is the last line of code called? I know there is a Javascript variable saveFile being used as a function, passing the parameter "file", but I've never seen the last line of code used before.

var saveFile = function (file) {

    // do the stuff   

} (typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content);

@JohnColeman This particular idiom in JavaScript programming does have a name, and it's called a "self-executing anonymous function". — Dai
– Dai, Commented Oct 25, 2019 at 22:04
@Dai -- but you are making a guess as to what OP means by "this". For all anyone knows, perhaps they are confused by typeof or the use of the logical operators. — John Coleman
– John Coleman, Commented Oct 25, 2019 at 22:04
@JohnColeman The OP wrote "What is the last line of code called?" - I assume they're asking about the } ( stuff ); syntax which is a hallmark of a SEAF. — Dai
– Dai, Commented Oct 25, 2019 at 22:06

Dai · Accepted Answer · 2019-10-25 22:08:28Z

3

It's a self-executing anonymous function - with a return value.

I know there is a Javascript variable saveFile being used as a function, passing the parameter "file", but I've never seen the last line of code used before.

This is not what is happening. saveFile is not the function - it's a variable that's assigned a value that is returned from that function.

Here's what the engine does when it runs that code:

Define the anonymous function.
The anonymous function is immediately followed by parenthesis which means the anonymous function should be invoked immediately.
The expression typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content is evaluated and the resulting value is used as the argument for the file parameter for the anonymous function.
The anonymous function runs its code and returns a value.
That return value is then stored in saveFile.

It's equivalent to doing this:

function anonymous_function( file ) {
    // do the stuff
}

var fileArg = typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content;

var saveFile = anonymous_function( fileArg );

edited Oct 25, 2019 at 22:08

answered Oct 25, 2019 at 22:02

Dai

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2 Comments

ricardoorellana Over a year ago

Great answer Dai!, just one comment I believe anonymous_function is a named function (A function declaration).

Dai Over a year ago

@ricardoorellana In OP's posted code, it's an anoynmous function. I posted code to show equivalent code if the function was a named function to make it easier to understand what's going on.

ricardoorellana · Accepted Answer · 2019-10-25 22:26:56Z

Let's break this into multiple steps:

You are creating a function expression and assigning the value to a variable called saveFile your function defines one parameter file:

var saveFile = function (file) {}
Your function gets executed immediately after creation and you pass an argument:

var saveFile = function (file) { // do the stuff } (typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content);
Your argument value depends on a short circuit evaluation:

(typeof self !== "undefined" && self || typeof window !== "undefined" && window || this.content);

Edit: Remember this, functions are first class objects in JavaScript, this simple means that you can do things like passing functions as arguments, declaring a function an assign it to a variable, add functions as values of objects, arrays and even return a function as a value.

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