这在 Javascript 变量 function 中叫什么？

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);

Answer 1

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:

1. Define the anonymous function.
2. The anonymous function is immediately followed by parenthesis which means the anonymous function should be invoked immediately.
3. The expression typeof self.== "undefined" && self || typeof window !== "undefined" && window || this.content typeof self.== "undefined" && self || typeof window !== "undefined" && window || this.content 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.
4. The anonymous function runs its code and returns a value.
5. 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 );

Answer 2

Let's break this into multiple steps:

1. 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) {}

2. 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);

3. 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.