Why Do Parentheses Create Encapsulated Anonymous Functions in JavaScript?

The Syntax of Encapsulated Anonymous Functions

In JavaScript, encapsulated anonymous functions are created by wrapping a function in brackets and executing it immediately:

(function(){
    // code here
})();

This syntax is used to avoid cluttering up the global scope with variables and functions.

Why This Works: (function(){})();

When parentheses surround a function declaration, the result is evaluated as a function expression. Function expressions allow for optional names, so this expression can run without a name.

Why This Doesn't Work: function(){}();

On the other hand, when there are no parentheses, JavaScript parses it as a function declaration. Function declarations require a name identifier, which is missing in this case.

Function Declaration vs. Expression

• Function declaration: function identifier(parameters) { ... }
• Function expression: (function identifier(parameters) { ... })

Function expressions can be named or unnamed, while function declarations must have a name.

Parentheses and Context

Parentheses indicate that the enclosed code is an expression. Whether it's a function declaration or expression depends on the context. Function declarations can appear in global scope or within the body of another function, while function expressions can only appear in expressions.

Example Ambiguity

0, function foo() {} // Function Expression
function foo() {} // Function Declaration

In this example, the parser determines whether it's a function declaration or expression based on the context. The expression can appear in expressions, while the declaration can only appear in specific locations.

Why Functions in Blocks Should Be Avoided

Functions inside blocks can lead to unexpected behavior due to variable scoping issues. For instance:

if (true) {
  function foo() {
    alert('true');
  }
} else {
  function foo() {
    alert('false!');
  }
}

foo(); // true? false? why?

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