e.log(foo); // 1
    (function foo() {
      foo = 1; // TypeError: Assignment to constant variable.
    })();
    

##  Examples

###  Using function expression

The following example defines an unnamed function and assigns it to ` x ` .
The function returns the square of its argument:

js

    
    
    const x = function (y) {
      return y * y;
    };
    

###  Using a function as a callback

More commonly it is used as a [ callback ](/en-
US/docs/Glossary/Callback_function) :

js

    
    
    button.addEventListener("click", function (event) {
      console.log("button is clicked!");
    });
    

###  Using an Immediately Invoked Function Expression (IIFE)

An anonymous function is created and called:

js

    
    
    (function () {
      console.log("Code runs!");
    })();
    
    // or
    
    !function () {
      console.log("Code runs!");
    }();
    

##  Specifications

Specification  
---  
[ ECMAScript Language Specification  
#  sec-function-definitions  ](https://tc39.es/ecma262/multipage/ecmascript-
language-functions-and-classes.html#sec-function-definitions)  
  
##  Browser compatibility

BCD tables only load in the browser  with JavaScript enabled. Enable
JavaScript to view data.

##  See also

  * [ Functions ](/en-US/docs/Web/JavaScript/Guide/Functions) guide 
  * [ Functions ](/en-US/docs/Web/JavaScript/Reference/Functions)
  * [ ` function ` ](/en-US/docs/Web/JavaScript/Reference/Statements/function)
  * [ ` Function ` ](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function)
  * [ Arrow functions ](/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions)

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