Why Must Immediately Invoked Function Expressions (IIFEs) Be Defined and Called on a Single Line?

Why Anonymous Functions Need to Be Invoked on the Same Line

It's common practice to create and invoke anonymous functions on the same line to encapsulate specific behaviors or protect data from global scope. However, understanding the underlying mechanics of this pattern can be enlightening.

Anonymous Function Syntax

An anonymous function is defined without an identifier. Using the function expression syntax:

(function(params) { ... })

Execution Trigger

To execute an anonymous function, it must be surrounded by parentheses. This triggers the JavaScript engine to treat the expression as a function call.

Syntax Distinction

Working:

(function(msg) { alert(msg); })('SO');

This syntax creates an anonymous function that alerts a message. The function is invoked immediately by the surrounding parentheses.

Not Working:

(function(msg) { alert(msg); });
('SO');

This syntax will fail because the semicolon after the function definition terminates the statement. The subsequent line is treated as a separate statement, attempting to pass a string to undefined.

Function Expression vs. Function Declaration

Function expressions differ from function declarations (e.g., "function name(...) {...}"). While declarations must have an identifier, function expressions can be anonymous. This allows them to be used in situations where naming is not necessary or desired.

In-Scope Identifier

Anonymous function expressions can have optional identifiers within their scope. However, these identifiers are only valid within the function body.

References

• [jQuery and $ questions](https://stackoverflow.com/questions/343336/jquery-and-questions)

By understanding these concepts, developers can confidently use anonymous functions to achieve encapsulation and maintain code organization.

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