Java script function

osc_ 4p2c0ecc 2021-01-21 10:29:22
java script function


What is a function ?

        A function is defined once but can be called or executed any number of times js Code . Functions sometimes have arguments , That is to say, when the function is called, it specifies the value of local variables . Functions often use these parameters to calculate a return value , This value also becomes the value of the function call expression . Generally speaking, a function is a section that will have a certain function js Encapsulation of code .

The concept of function declaration

        Function is a core concept for any language . You can encapsulate any number of statements through functions , And it can be anywhere 、 Call execution at any time .ECMAScript The function in the function Keyword to declare , This is followed by a set of parameters and function bodies .
example :

function box(){
 // Functions without parameters 
alert(' Only functions are called , Statement will be executed ');
}

Defined function

Format 1 :

<script type="text/javascript">
/*function Function name ([ Parameter list ]){
The body of the function ;
[return< expression >;]
}*/
// for example :
function show(){

console.log(' This is my first function !');
}
</script>

Format two :

<script type="text/javascript">
/*var Variable name = function( Parameter list ){
The body of the function ;
[ return < expression >; ]
}*/
// for example :
var show2 = function (){

console.log(' This is my second function !');
}
</script>

Format three :
Immediate execution function (IIFE), With a few , Call itself automatically , It can't be called anywhere else , Commonly used in JS Library or JS Encapsulation or closure of plug-ins .

<script type="text/javascript">
/*(function ( Parameter list ){
The body of the function ;
[ return< expression >; ]
}) ( Argument list )*/
// for example :
(function (){

var rs = 1 + 2;
console.log(rs);
}
</script>

Function call

Functions don't execute automatically (IIFE( Execute the function expression immediately ) With the exception of ), Must be called to execute .
Call without parameters :

<script type="text/javascript">
// No parameter 
function show(){

console.log(' This is my first function !');
}
var show2 = function (){

console.log(' This is my second function !');
}
show1();// If you don't pass the parameters , The brackets here can't be omitted .
show2();
</script>

Call with parameters , The number of parameters matches , If the number of real parameters exceeds the number of formal parameters , No mistake. , The argument is passed to the formal parameter once ; If the number of formal parameters exceeds the number of actual parameters , A formal parameter has no value that matches an argument , Then the default value of the formal parameter is undefined.

<script type="text/javascript">
// With parameters 
function sum1(a,b){

var rs = a + b;
console.log(rs);
}
var sum2 = function (x,y){

var rs = x - y;
console.log(rs);
}
sum1(10,20);
sum2(20,-30);
</script>

return

Functions can be return Return results , If return No results returned , Represents a call to an end function , And return to the use of .

<script type="text/javascript">
function max(a,b){

var result = a > b ? a : b ;
return result;
}
console.info(max(10,20))
</script>
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