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What does the exclamation mark do before the function?
If you look at the source code for KnockoutJS 2.1.0 you will see a code structure like this start on line 7:
!function(factory) { ... }(factoryDefinition); The not operator causes this expression to evaluate to true rather than undefined, but why bother?
This is a concise way to form an immediately executed function expression.
Traditionally, people have used these two forms
(function(){ }()); // Recommended by Crockford (function(){ })(); // What most people use If you try to just use
function(){ }(); // Syntax error it will be a syntax error, because it is interpreted as a function declaration rather than an expression. This is why you would need to wrap the function in parentheses.
But if you put a unary operator before the function declaration, you don't have to add a cosing parentheses, and it chops off one character of the code, which is a (very) tiny performance benefit. There are several unary operators that can be used for this same purpose
!function(){ }(); ~function(){ }(); -function(){ }(); +function(){ }(); Because if you don't do something then it looks like a syntax error:
function(factory) { ... }(factoryDefinition); Try it.
It's necessary to get the parser to a point where it expects an expression, so that the function keyword is recognized in that context. Otherwise, when the parser sees function as the first token in a statement, it expects a simple function declaration, and that can't be followed by a parenthesized argument list.
There are various alternatives, around each of which cluster various opinions.
function(message){console.log(message)}('hello'); result: hello
...)
function(message){console.log(message)}('hello')