function declarations and function expressions in "javascript" javascript

JavaScript has a lot of interesting uses and can be found in Google Code search, for example:

<script>~function () {    alert ("Hello, World");} ();</script>

Try to know that the meaning of this code is to declare a function, and then immediately execute, because the scope of variables in JavaScript is based on function, so this can avoid the variable pollution, but here the bitwise operator "~" At first glance let a person cannot touch the mind, If you remove it and run it, you will get an error: SyntaxError.

Before explaining why, let's first clarify two concepts in javascript: function declaration and function expression:

Let's take a look at what a function declaration looks like:

<script>function () {    alert ("Hello, World");}; function foo () {    alert ("Hello, World");}; </script>

Let's look at what the function expression is:

<script>var foo = function () {    alert ("Hello, World");}; </script>

Now look back to the article at the beginning of the question, why the removal of the bitwise operator "~" After the error, because from a syntactic perspective, JavaScript is not allowed to use the parentheses directly after the function declaration, and the function expression does not have this restriction, by adding a "~" operator before the function declaration, It is possible for the parser to think of the latter as a function expression, as well as the addition of "!,+,-" operators in front of the function declaration.

So why don't we use the following function expressions?

<script>var foo = function () {    alert ("Hello, World");} ();</script>

Although there is no problem from the point of view of syntax parsing, the above code is flawed, introducing a variable that may contaminate the existing operating environment and pose potential problems.

The use of the bitwise operator "~" is a bit artifice, in fact, the function declaration with parentheses to make it easier to read:

<script> (function () {    alert ("Hello, World");}) ();</script>

Understand the principle, no matter what writing, will not confused again.

This entry is posted in Technical and tagged Javascript by Lao Wang. Bookmark the permalink.

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function declarations and function expressions in "javascript" javascript