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JS FunctionsLesson #223

Self-Invoking Functions

An IIFE is short for an Immediately Invoked Function Expression.

Immediately Invoked Function Expressions

An IIFE is short for an Immediately Invoked Function Expression.

An IIFE is a function that invokes itself when defined.

What Is an IIFE?

Normally, a function runs only when it is called.

An IIFE runs automatically when the JavaScript engine reads it (compiles it).

Example

Example Code

javascript

Try it Yourself

(function () {

  // Code to run immediately

})();

An IIFE function is defined and executed at the same time.

When is IIFE Used?

To create a private scope
To run setup code once
In older JavaScript code

Avoid Polluting the Global Scope

Everything inside an IIFE is private to that function.

Variables inside an IIFE cannot be accessed from outside.

Examples

Example Code

javascript

Try it Yourself

(function () {

  let x = 10;

})();

// x is not accessible here

Simple IIFE

Example

Example Code

javascript

Try it Yourself

(function () {

  let text = "Hello! I called myself.";

})();

The function above is also called an anonymous self-invoking function (function without name).

How an IIFE Works

Function expressions can be made self-invoking.

A self-invoking function expression is invoked (started) automatically, without being called.

Function

Example Code

javascript

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function () {

   // Code to run

};

Parentheses around the function tell JavaScript to treat the function as an expression.

Expression

Example Code

javascript

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(function () {

   // Code to run

});

Function expressions will execute automatically if the expression is followed by ().

Final Syntax

Example Code

javascript

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(function () {

   // Code to run

})();

You can only self-invoke a function expression. > You can not self-invoke a function declaration.

IIFE with Parameters

You can pass arguments into an IIFE.

Example

Example Code

javascript

Try it Yourself

(function (name) {

  let text = "Hello " + name;

})("John Doe");

IIFE with Return Value

An IIFE can return a value, which can be stored in a variable.

Example

Example Code

javascript

Try it Yourself

let result = (function () {

      return 5 + 5;

    })();

Arrow Function IIFE

Arrow functions can also be used as IIFEs.

Example

Example Code

javascript

Try it Yourself

(() => {

  let text = "Hello! I called myself.";

})();

Arrow Function IIFE with Parameter

Example

Example Code

javascript

Try it Yourself

((name) => {

  let text = "Hello " + name;

})("John Doe");

IIFE in Modern JavaScript

Today, JavaScript modules often replace the need for IIFEs. Modules have their own scope by default.

Example

Example Code

html

Try it Yourself

let x = 10;

The variable x is scoped to the module and not global.

Named Function Expression IIFE

You can give an IIFE a name:

Example

Example Code

javascript

Try it Yourself

(function greet() {

  let text = "Hello! I called myself.";

})();

greet(); // &#9940; ReferenceError

But, the name greet exists only inside the function itself, not in the outer/global scope.

Why use a named IIFE?

For self-recursion functions (calling itself repeatedly):

Example

```javascript (function factorial(n) {

if (n

Self Invocing Functions were heavily used before ECMAScript 6 > Today, JavaScript let, const, > and modules have reduced the need for them. > But they can still be useful for: > Running setup code that should immediately > Creating private variables and private scope > Avoiding global scope pollution > Working with older JavaScript code that rely on them

When Not to Use IIFE

When using modern JavaScript modules
When simpler code is sufficient

Quiz > Why were IIFEs commonly used before JavaScript modules existed?

Common Mistakes

### Forgetting the Final Parentheses Without (), the function will not run.
### Confusing IIFE with Normal Functions IIFEs cannot be called again.
### Overusing IIFEs Modern JavaScript often does not need them.