Introduction to Javascript Part 5 js function page 1/2_Basic knowledge

1, Function:
function is a js code that is defined once but can be called multiple times.
When a function is called by an object, then the function is called a method of the object.

function cssrain( x , y) 
{ 
 //code 
}

Explanation:
cssrain : is the function name;
() : is the operator;
x, y : is the parameter;

2, the return value of the function:

function a(x){ 
    document.write(x); 
} 
function b(y){ 
   document.write(y); 
   return y; 
} 
alert( a(1) )  //因为没写return，所以返回undefined 
alert( b(2) )

3 function statement and function literal:

function f(x) { return x * x  ;}   // 
var f  = function(x){ return  x * x ;}   //

The first one is created by the function statement, The second one is to directly define an expression using a function literal. Of course, in this way, an anonymous function is created.
Although the direct variable can be anonymous, you can also specify a function name;
For example:

var f  = function fact(x){ return  x * fact(x-1) ;}   //这样做的好处; 调用自身非常爽。

4 Function naming:
function like_this(){}
or function likeThis(){} // Camel case

5 Function parameters:
Since js is a loosely typed language, the parameters do not need to specify any data type. Parameters can be more or less,
For example: function x(a,b){} //We wrote 2 parameters
If we pass 3 parameters, js will automatically ignore the extra ones/
Example:

function x(a,b){ 
 document.write(a+ "  "+b); 
}  
x(1,2,3,4);

What happens if we only pass one parameter?

function x(a,b){ 
 document.write(a+ "  "+b); 
}  
x(1);

We found that undefined was output, so js will assign the less to undefined;
This may cause program errors.
Solution:

function x(a,b){ 
var b = b || " ";  // 这个是或运算符，如果前面的b为undefined，也就是false，他会取后面的空字符 
 document.write(a+ "  "+b); 
}  
x(1);