Functions in JS

function

Realize the reuse of code

Two ways to declare

• Use function keywords to customize functions ( Name the function )
• Function expression ( Anonymous functions )

var  Variable name  = function () {}

Use steps

• Declare functions
• Call function

function  Function name  （） {  The body of the function  }

Be careful ：

1. function Key to declare function , All lowercase
2. A function is to do something , Function names usually use verbs
3. The function does not call and does not execute itself

Call function

Function name （）;

Encapsulation of functions

• Function encapsulation is to pass one or more functions Function to encapsulate , It only provides a simple function interface to the outside world
• Similar to express packaging

The parameters of the function

Formal parameters and actual parameters

function  Function name  （ Shape parameter 1, Shape parameter 2...） {
    // The body of the function 
}
 Function name  ( Actual parameters 1, Actual parameters 2);

When you declare a function , You can add some parameters after the parentheses , These parameters are called Shape parameter , When the function is called , You also need to pass the corresponding parameters , These parameters are called Actual parameters

Parameter function ： Internal function Some values are not fixed , You can use parameters in Pass... When calling a function Different values go in

Be careful ：

• Multiple parameters are separated by commas
• Formal parameters can be regarded as undeclared variables

The number of function parameters and arguments does not match

If the number of arguments is more than the number of formal parameters , Will get the number of formal parameters

If the number of arguments is less than the number of formal parameters , Redundant formal parameters are defined as undefined, The end result is NaN

Summary

• Functions can take parameters or no parameters
• When you declare a function , In parentheses of function name are formal parameters , The default value is undefined
• When you call a function , Inside the parentheses of the function name is the argument
• Multiple parameters are separated by commas
• The number of formal parameters can not match the number of actual parameters

The return value of the function

return sentence

• A function is just a function , The final result needs to be returned to the caller function name of the function （） adopt return
• As long as the function encounters return, Just return the result to the caller of the function   Function name （） = return The latter result

In actual development , Often use a variable to receive the return value of a function

return Terminate function

return The subsequent code is not executed

return The return value of

return Only one value can be returned , If you separate multiple values with commas , Subject to the last one

If the function has return, return return Value after , If the function doesn't have return Then return to undefined

break return continue The difference between

• break： End the current cycle
• continue： Jump out of this cycle , Continue with next cycle
• return： Not only exit the loop body , And return to return Value in statement , At the same time, you can also end the code in the current function body

arguments Use

When it is uncertain how many parameters are passed , It can be used arguments obtain , stay js in ,arguments It is a of the current function Built-in objects . All functions have a built-in arguments object ,arguments object Stores all arguments passed

arguments The display form is a pseudo array , It can be traversed , It has the following characteristics ：

• have length attribute
• Store data by index
• Does not have an array push、pop Other methods

Only functions have arguments object

Function expression declares function

1. fun Is a variable name. , It's not a function name
2. Function expressions are declared in much the same way as variables , But the value stored in the variable , What is stored in the function expression is the function
3. Function expressions can also pass arguments