64 lines
2.2 KiB
Markdown
64 lines
2.2 KiB
Markdown
|
---
|
||
|
|
title: Immediately Invoked Functions Expressions(IIFEs)
|
||
|
|
---
|
||
|
|
## Function Statement
|
||
|
|
|
||
|
|
A function created with a function declaration is a Function object and has all the properties, methods and behavior of Function objects.
|
||
|
|
Example:
|
||
|
|
|
||
|
|
```javascript
|
||
|
|
function statement(item){
|
||
|
|
console.log('Function statement example '+ item);
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
## Function Expression
|
||
|
|
|
||
|
|
A function expression is similar to function statement except that function name can be omitted to create anonymous functions.
|
||
|
|
Example:
|
||
|
|
|
||
|
|
```javascript
|
||
|
|
var expression = function (item){
|
||
|
|
console.log('Function expression example '+ item);
|
||
|
|
}
|
||
|
|
```
|
||
|
|
|
||
|
|
## Immediately Invoked Functions Expressions
|
||
|
|
|
||
|
|
A soon as function is created it invokes itself doesn't need to invoke explicitly.
|
||
|
|
In the below example variable iife will store a string that is returned by the function execution.
|
||
|
|
|
||
|
|
```javascript
|
||
|
|
var iife = function (){
|
||
|
|
return 'Immediately Invoked Function Expressions(IIFEs) example ';
|
||
|
|
}();
|
||
|
|
console.log(iife); // 'Immediately Invoked Function Expressions(IIFEs) example '
|
||
|
|
```
|
||
|
|
|
||
|
|
The statement before IIFE should always end with a ; or it will throw an error.
|
||
|
|
|
||
|
|
**Bad example**:
|
||
|
|
```javascript
|
||
|
|
var x = 2 //no semicolon, will throw error
|
||
|
|
(function(y){
|
||
|
|
return x;
|
||
|
|
})(x); //Uncaught TypeError: 2 is not a function
|
||
|
|
```
|
||
|
|
|
||
|
|
## Why use Immediately Invoked Functions Expressions?
|
||
|
|
|
||
|
|
```javascript
|
||
|
|
(function(value){
|
||
|
|
var greet = 'Hello';
|
||
|
|
console.log(greet+ ' ' + value);
|
||
|
|
})('IIFEs');
|
||
|
|
```
|
||
|
|
|
||
|
|
In above example when javascript engine execute above code it will create global execution context when it sees code and create function object in memory for IIFE.
|
||
|
|
And when it reaches on line `46` due to which function is Invoked a new execution context is created on the fly and so greet variable goes into that function execution context not into the global this is what makes it unique.
|
||
|
|
`This ensures that code inside IIFE does not interfere with other code or be interfered by another code` and so code is safe.
|
||
|
|
|
||
|
|
#### More Information
|
||
|
|
[Immediately-invoked function expression on Wikipedia](https://en.wikipedia.org/wiki/Immediately-invoked_function_expression)
|
||
|
|
[What does the leading semicolon in JavaScript libraries do?](https://stackoverflow.com/questions/1873983/what-does-the-leading-semicolon-in-javascript-libraries-do)
|