JS execution environment and scope analysis

Execution environment: defines other data that variables or functions have access to, and determines their respective behaviors. Each execution environment has a variable object associated with it, and all variables and functions defined in the environment are stored in this object.

Global execution environment: It is the most peripheral execution environment. In web browsers, the global execution environment is considered to be the window object, so all global variables and functions are created as properties and methods of the window object. After all code in an execution environment has been executed, the environment is destroyed, along with all variable and function definitions saved in it.

Execution flow mechanism: Each function has its own execution environment. When the execution flow enters a function, the function's environment is pushed into an environment stack. When the function is executed, the stack pops its environment and returns control to the previous execution environment.

! ! When code is executed in an environment, a scope chain of variable objects is created. The purpose of the scope chain is to ensure ordered access to all variables and functions that the execution environment has access to. The front end of the scope chain is always the variable object of the environment where the currently executing code is located. If this environment is a function, its active object is used as a variable object. The active object initially contains only one variable, the arguments object. The next variable object in the scope chain comes from the containing (external) environment, and the variable object after that comes from the next containing environment. In this way, one continues to the global environment; the variable object of the global execution environment is always the last object in the scope chain.

The internal environment can access all external environments through the scope chain, but the external environment cannot access any variables and functions in the internal environment.

Extend scope chain

Execution environment type: global and local.

Some statements can temporarily add a variable object at the front end of the scope chain, and the variable object will be removed after the code is executed.

• The catch block of the try-catch statement (will create a new variable object, which contains the declaration of the thrown error object);

• with statement (will add the specified object to the scope).

No block-level scope

For JavaScript, the variable i created by the for statement will still exist outside the loop even after the for loop execution ends. in the execution environment.

1. Variable declaration

Variables declared using var will automatically be added to the closest environment. Inside a function, the closest environment is the function's local environment; in a with statement, the closest environment is the function environment. If a variable is initialized without a var declaration, the variable will be automatically added to the global environment.

2. Query Identifiers

When an identifier is referenced for reading or writing in an environment, a search must be performed to determine what the identifier actually represents. The search process starts at the front of the scope and works its way up to find identifiers that match the given name.

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