Why Do We Invoke Anonymous JavaScript Functions on the Same Line They're Defined?

Invoking Anonymous Functions on the Same Line: Why It Matters

In the context of closures, anonymous functions are often invoked on the same line as they are defined. This practice raises questions about the underlying mechanisms and the reasons behind it.

In Javascript, functions can be defined using three methods. Firstly, the Function constructor creates functions using the new keyword. Secondly, function declarations use the function keyword and a function name. Lastly, function expressions assign functions to variables without specifying a name.

In the provided code, an anonymous function is defined using a function expression:

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

However, if the semicolon after the function definition is dropped, the code fails:

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

This is because without the semicolon, the code is interpreted as two separate statements:

1. An anonymous function declaration.
2. A call to that function.

This is not semantically correct, as the function is not defined before it is called. To avoid this error, the semicolon must be present.

Alternatively, the function expression can be defined and invoked as follows:

var myFunc = function(msg){alert(msg);}
myFunc('SO');

In this case, the function is assigned to a variable before it is invoked. However, this approach requires a named function, which may not always be desired.

The practice of invoking anonymous functions on the same line allows for immediate execution of the function without the need for a named variable. This is particularly useful when encapsulating code that needs to be executed only once. Additionally, it helps prevent naming conflicts and reduces the number of global variables in the codebase.

For further understanding of anonymous functions and function expressions, refer to the ECMA script specification (Section 13 Function Definition) or additional resources like "jQuery and $ questions" or the examples provided in the code snippets above.

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