JavaScript scope chain 1: Scope chain Definition

Data in the execution context (variables, function declarations, and function parameters) are stored as attributes in the variable object. Variable

Data in the execution context (variables, function declarations, and function parameters) are stored as attributes in the variable object. The variable object is created every time it enters the context and is filled with the initial value. The value is updated in the code execution phase.

This chapter focuses on more details that are directly related to the execution context. This time we will mention an issue-the scope chain.

If you want to briefly describe and display the focus, most of the scope chains are related to internal functions.

We know that ECMAScript allows the creation of internal functions, and we can even return these functions from the parent function.

var x = 10; function foo() {   var y = 20;   function bar() {    alert(x + y);  }   return bar; } foo()(); // 30

In this way, it is obvious that each context has its own variable object: for a global context, it is the global object itself; for a function, it is the activity object.

The scope chain is the list of all variable objects (including parent variable objects) in the internal context. This link is used for variable query. In the preceding example, the scope chains of bar context include AO (bar), AO (foo), and VO (global ).

However, let's take a closer look at this issue.

Let's start with the definition and further discuss the example.

The scope chain is related to an execution context. The variable object chain is used for Variable Search in identifier parsing.

The scope chain of the function context is created when a function is called, including the activity object and the [[scope] attribute inside the function. Next we will discuss in more detail the [[scope] attribute of a function.

The following is a diagram in the context:

ActiveExecutionContext = {VO :{...}, // or AO this: thisValue, Scope: [// Scope chain // list of all variable objects // for identifiers lookup]};

Its scope is defined as follows:

Scope = AO + [[Scope]]

This association and identifier parsing process will be discussed below, which is related to the function lifecycle.

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

This article is available at http://www.nowamagic.net/librarys/veda/detail/1675.