JavaScript function

##Basic concept of function

is a program instruction (statement) that completes a certain function Sets are called functions.

Classification of JavaScript functions

1. Custom functions (functions written by ourselves), such as: function funName(){}

　2. System functions (functions that come with JavaScript), such as alert function.

How to call the function

1. Ordinary call: functionName (actual parameter...)

2. Pass Point to the variable of the function to call:

var myVar=function name;

myVar(actual parameter...);

Function return value

　1. When a function has no clear return value, the returned value is "undefined".

　2. When a function has a return value, it returns whatever the return value is.

    var str="window.alert('好好學(xué)習(xí)');";
    eval(str);//eval() 函數(shù)可計(jì)算某個(gè)字符串，并執(zhí)行其中的的 JavaScript 代碼。
    /*自定義函數(shù)*/
    function test(str){
        alert(str);
    }
    window.alert(test);//輸出test函數(shù)的定義
    //函數(shù)的調(diào)用方式1
    test("好好學(xué)習(xí)");
    //函數(shù)的調(diào)用方式2
    var myFunction=test;
    myFunction("天天向上");
    window.alert(myFunction);
    /*當(dāng)函數(shù)無(wú)明確返回值時(shí)，返回的也是值 "undefined"*/
    var retVal=test("test");//test函數(shù)執(zhí)行完之后，并沒(méi)有返回值，因此retVal變量接收到的返回值結(jié)果是undefined
    alert("retVal="+retVal);//輸出undefined