JavaScript function declaration and function expression

When I read another blog today, I found thatArticleI have been talking about function declarations and function expressions,

I was a little dizzy, and I sorted it out later. In fact, these two concepts have been used before, but they are not noticed!

1.

Function declaration:

 
FunctionA () {alert ("Hello World")}//This is a function declaration.
 

Function expression:

 
VaRA =Function() {Alert ("Hello World")}//This is a function expression.
 

So what are the differences between them?

The parsing of the function declaration is in the pre-execution stage, that is, the browser is ready to executeCodeTimeAnd function expressions, just like defining other basic types of variables, will be parsed only when a certain sentence is executed..

2. The following are some weird functions.

 
~FunctionA () {alert (1111);}();
 

 
+FunctionA () {alert (1111);}();
 

However ~ + After removal, they will report a syntaxerror. This is because from the perspective of Syntax Parsing, JavaScript cannot directly use parentheses after function declaration, while function expressions do not have this restriction. By adding "~ "," + ","-","! "Operator, you can let the syntax parser regard it as a function expression, but the performance of different operators is not the same. If you want to know, refer to the following blog: http://www.swordair.com/blog/2011/10/714 /. In fact, the above functions can be written in this way.

 
VaRA =Function() {Alert (1111);}();
 

However, another problem occurs because you define a variable that will pollute the current running environment and bring potential risks.

However, if you do not like to add "~ "," + ","-","! "Operator, which can be written as follows. In fact, this is also a common method for many JS libraries.

(FunctionA () {alert (1111);})();
 

I believe that those who have read prototype, jquery, and other libraries must have seen this method.