JS scope and pre parsing

Scope

Qualified by this name Code scope of availability That's the name Scope . The use of scope increases the limitations of program logic , Enhance program reliability , Reduce name conflicts

It is the code name that plays a role and effect in this range

JS The scope is divided into （es6 Before ）

• Global scope
• Local scope

Global scope

Whole script Label or separate one js file

Local scope （ Function scope ）

Inside the function , It's a local scope , It only works within functions

Scope of variable

Depending on the scope , Variables can be divided into two types ：

• Global variables
• local variable

Global variables

stay Under global scope The variable of

Variables that are not declared inside a function are also global variables

local variable

stay Under local scope The variable of , Or in Internal function The variable of

Be careful ：

If it's inside a function , Variables that do not declare direct assignment are also global variables

The formal parameters of a function can also be regarded as local variables

1. In terms of execution efficiency , Global variables are destroyed only when the browser is closed , Compare memory

2. Local variables are destroyed after the program is executed , Saving memory

Block level scope {}

stay {} The variables inside , You can call

Scope chain

• As long as the time code , There is at least one scope
• The local scope written inside the function
• If there are functions in the function , Then a scope can be born in this scope
• According to the mechanism that internal functions can access external function variables , Use chain search to determine which data can be accessed by internal functions , It's called the scope chain

JS Pre parse

js The code is generated by js Parser to execute ,js The parser is running js The code is divided into two steps ： Pre parse and Code execution

js The engine will turn js Everything in it var and function Promote to the front of the current scope , Then execute the code in the order of code writing

Pre parsing is divided into variable pre parsing （ Variable Promotion ） And function pre parsing （ Function enhancement ）

• Variable promotion is to put all Variable declarations Promote to the front of the current scope , Do not promote assignment operations
• Function promotion is to put all Function declaration Promote to the front of the current scope , Don't call functions

Be careful ： The promotion of a function expression must be written below the function expression