How Do I Invoke an Anonymous JavaScript Function on the Same Line?

Invoking Anonymous Functions on the Same Line

In JavaScript, an anonymous function is a function without an identifier. It can be created using a function expression, which involves assigning the function to a variable or using it directly.

Understanding how anonymous functions work is crucial. When you write a function expression:

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

you create an anonymous function. However, you must execute the function expression immediately by wrapping it in parentheses and adding arguments within the parentheses:

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

Attempting to separate the function expression from the execution, as shown below, will not work:

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

This is because the function expression creates a function object. To execute the function, you must invoke it by following it with parentheses and arguments. Omitting the parentheses after the function expression prevents its execution.

Here's an alternative explanation from the ECMA Script specification. Function definitions come in three forms: using the Function constructor, using a function declaration, or using a function expression.

Function Expression

A function expression allows you to create an anonymous function by omitting the identifier:

function (msg) { return a + b; }

Armed with this knowledge, let's break down your original code:

(function (msg) {
    // ...
})();

This code creates an anonymous function and executes it immediately by wrapping it in parentheses. The function scope is now closed, and you can no longer access its variables or methods from outside the function.

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