本文实例讲述了JS实现的判断方法、变量是否存在功能。分享给大家供大家参考，具体如下：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>Title</title></head><body>//http://www.vevb.com/article/67551.htm//判断变量i是否存在 typeof(i)=="undefined"<script> /*---------------------------判断函数是否存在-------------------------------*/ function isExitsFunction(funcName) { try { if (typeof(eval(funcName)) == "function") { return true; // funcName(); } } catch (e) { console.log(eval(funcName) + "+++++++++++++++++我异常了!!!!!!!!"); } return false; } /*--------------------------------判断是否存在指定变量 -----------------------------------------*/ function isExitsParamsVariable(variableName) { try { console.log("variableName.length===" + variableName.length); if (variableName.length == 0) { console.log(variableName + "===value has no params");//""：length为0 return false; } else { console.log(variableName + "======value has params");//0:length为undefined return true; } } catch (e) { console.log(variableName + "----我异常了!!!!!!!!");//null,undefined,未赋值的a } return false;//null,undefined,未赋值的a } /*---------------------------------判断是否undefined--------------------------------*/ function isExitsVariable(variableName) { console.log("typeof variableName====" + typeof(variableName)); try { if (typeof(variableName) == "undefined") { console.log(variableName + "===value is undefined");//undefined,未赋值的a return false; } else { console.log(variableName + "=======value is true");//null,0,"" return true; } } catch (e) { console.log(variableName + "-------我异常了........"); } return false; } /*-------------------------------------------------测试数据---------------------------------------------*/ var a;//声明未初始化，没有长度 console.log("isExitsParamsVariable(a)" + isExitsParamsVariable(a)); console.log("isExitsVariable(a)" + isExitsVariable(a)); console.log("--------------------------------------------------") var b = undefined;//没有长度 console.log("isExitsParamsVariable(b)===" + isExitsParamsVariable(b)); console.log("isExitsVariable(b)===" + isExitsVariable(b)); console.log("--------------------------------------------------") var c = null;//没有长度 console.log("isExitsParamsVariable(c)===" + isExitsParamsVariable(c)); console.log("isExitsVariable(c)===" + isExitsVariable(c)); console.log("--------------------------------------------------") var d = 0;//长度undefined console.log("isExitsParamsVariable(d)===" + isExitsParamsVariable(d)); console.log("isExitsVariable(d)===" + isExitsVariable(d)); console.log("--------------------------------------------------") var e = "";//长度为0 console.log("isExitsParamsVariable(e)====" + isExitsParamsVariable(e)); console.log("isExitsVariable(e)===" + isExitsVariable(e)); console.log("--------------------------------------------------") /*未定义声明 f 则log会报错：Uncaught ReferenceError: f is not defined ，不会执行两个判断方法*/ console.log("isExitsParamsVariable(f)====" + isExitsParamsVariable(f));//f:undefined console.log("isExitsVariable(f)===" + isExitsVariable(f));</script></body></html>希望本文所述对大家JavaScript程序设计有所帮助。
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