JavaScript Functions 1: function declaration

Next we will introduce a very common ECMAScript object-function (function

The following describes a very common ECMAScript object-function ), we will explain in detail how various types of functions affect context variable objects and what each function's scope chain contains, and answer questions such as the following: what are the differences between the functions declared below? (If so, what is the difference ).

var foo = function () {  ...};

Usual method:

function foo() {  ...}

Or, why should the following functions be enclosed in parentheses?

(function () {  ...})();

But we still need to look at them one by one, starting with the function type:

There are three function types in ECMAScript: function declaration, function expression, and function created by the function constructor. Each type has its own characteristics.

Function Declaration

Function declaration (FD) is a function:

1. There is a specific name
2. Position in the source code: either in the Program level or in the FunctionBody of other functions
3. Create in the context stage
4. Variable objects
5. The following statement

function exampleFunc() {  ...}

The main feature of this function type is that they only affect variable objects (that is, variable objects stored in the context VO ). This feature also explains the second important point (it is the result of the properties of the variable object) -- they are available in the code execution stage (because FD already exists in VO at the context stage-before code execution ).

For example, a function is called before it is declared)

foo(); function foo() {alert('foo');}

Another key point is the second point in the above definition-the position of the function declaration in the source code:

// The function can be declared in the following places: // 1) function globalFD () {// 2) in the global context or function innerFD () in the function body of a function () {}}

Only the two positions can declare the function, that is, it cannot be defined in the expression position or a code block.

Additional reading

The topic list of this article is as follows:

1. How should we understand the working principle of the JavaScript engine?
2. JavaScript exploration: the importance of writing maintainable code
3. JavaScript exploration: exercise caution when using global variables
4. JavaScript exploration: var pre-parsing and side effects
5. JavaScript exploration: for Loop (for Loops)
6. JavaScript exploration: for-in loop (for-in Loops)
7. Exploring JavaScript: Prototypes is too powerful
8. JavaScript: eval () is the devil"
9. JavaScript exploration: Using parseInt () for Numerical Conversion
10. Exploring JavaScript: Basic coding specifications
11. JavaScript exploration: function declaration and function expression
12. JavaScript exploration: Name function expressions
13. JavaScript: function name in the debugger
14. JavaScript: JScript Bug
15. JavaScript exploration: Memory Management of JScript
16. Exploring JavaScript: SpiderMonkey's quirks
17. JavaScript exploration: an alternative solution to naming function expressions
18. JavaScript exploration: Object
19. JavaScript exploration: Prototype chain
20. JavaScript exploration: Constructor
21. JavaScript probing: executable context Stack
22. Execution context 1: Variable object and activity object
23. Execution context 2: Scope chain Scope Chains
24. Execution context 3: Closure Closures
25. Execution context 4: This pointer
26. Exploring JavaScript: Powerful prototype and prototype chain
27. JavaScript Functions 1: function declaration
28. JavaScript function 2: function expressions
29. JavaScript function 3: function expressions in a group
30. JavaScript function 4: function Constructor
31. JavaScript variable object 1: VO Declaration
32. JavaScript variable object 2: VO in different execution contexts
33. JavaScript variable object 3: two stages of execution Context
34. JavaScript variable object IV: Variables
35. Property of the JavaScript variable object __parent _
36. JavaScript scope chain 1: Scope chain Definition
37. JavaScript scope chain 2: function Lifecycle
38. JavaScript scope chain 3: Scope chain features
39. JavaScript closure 1: Introduction to closures
40. JavaScript closure 2: Implementation of closure
41. JavaScript closure 3: Closure usage

Address of this article: http://www.nowamagic.net/librarys/veda/detail/1662,welcome.