JS foundation 2

function ： Encapsulation of a piece of code , It is mainly used to realize a certain function （ Function is the embodiment of code encapsulation and reuse ）

Definition

declarative function Function name （）{ The body of the function }
call ： Function name （）
notes ： Functions do not actively execute , Need to call
expression let|var Function name =functtion(){ The body of the function }
call : Function name （）
notes ： Declarative calls can be made before a function is defined , Expressions cannot

Definition of functions with parameters

Implement known data function code
In the parentheses that define the function , Fill in formal parameters , Multiple are separated by commas
Fill in the arguments in the parentheses of the calling function

Relationship between formal and actual parameters

Formal and actual parameters are in number , The order , Data types should be consistent
If the argument is less than the formal parameter , The parameter value is undefined

Variable scope

Global variables ： Global availability . The function passes through var Defined , Or variables defined directly without keywords inside the function body
local variable ： Variables defined inside a function body

The return value of the function

When you need the result of the function for the next operation , The return value is required when the return is explicitly required
function Function name （）{
The body of the function
return Variable , value , expression ;
}
call ： Function name （）
notes ：return Once executed , Later code will not be executed ,
Function if not return, So the return value of the function is underfined

precompile

Function before line by line execution , Will precompile first
Variable Promotion ： Promote the code declared by the variable to the front of the scope

• var Variables defined
• Declaratively defined functions
• return Subsequent statements
• if Inside the sentence （ Even if the conditions don't hold , It will also increase the variable ）

arguments

1. Function's own object ： Record the correlation of function parameters
2. Get the number of function arguments arguments.length
3. Get a specific argument arguments[ The index of the corresponding argument ]
   notes ： Index from 0 Start

Recursive function

Recursive function ： The function calls itself
notes ： Recursion must have an end

Common events

1. Get element node document.getElementById('id name ')
2. Get element properties Element nodes . Property name
3. Set element properties Element nodes . Property name = Property value
4. The binding event Element nodes .on Event type =function (){ To execute code }
   Common event types

event	meaning
onclick	Click on
onlode	Page loading complete
onmouseover	Mouse hovering
onmouseout	Mouse removal