本文實例講述了JS實現的判斷方法、變量是否存在功能。分享給大家供大家參考，具體如下：
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>Title</title></head><body>//http://www.survivalescaperooms.com/article/67551.htm//判斷變量i是否存在 typeof(i)=="undefined"<script> /*---------------------------判斷函數是否存在-------------------------------*/ function isExitsFunction(funcName) { try { if (typeof(eval(funcName)) == "function") { return true; // funcName(); } } catch (e) { console.log(eval(funcName) + "+++++++++++++++++我異常了!!!!!!!!"); } return false; } /*--------------------------------判斷是否存在指定變量 -----------------------------------------*/ function isExitsParamsVariable(variableName) { try { console.log("variableName.length===" + variableName.length); if (variableName.length == 0) { console.log(variableName + "===value has no params");//""：length為0 return false; } else { console.log(variableName + "======value has params");//0:length為undefined return true; } } catch (e) { console.log(variableName + "----我異常了!!!!!!!!");//null,undefined,未賦值的a } return false;//null,undefined,未賦值的a } /*---------------------------------判斷是否undefined--------------------------------*/ function isExitsVariable(variableName) { console.log("typeof variableName====" + typeof(variableName)); try { if (typeof(variableName) == "undefined") { console.log(variableName + "===value is undefined");//undefined,未賦值的a return false; } else { console.log(variableName + "=======value is true");//null,0,"" return true; } } catch (e) { console.log(variableName + "-------我異常了........"); } return false; } /*-------------------------------------------------測試數據---------------------------------------------*/ var a;//聲明未初始化，沒有長度 console.log("isExitsParamsVariable(a)" + isExitsParamsVariable(a)); console.log("isExitsVariable(a)" + isExitsVariable(a)); console.log("--------------------------------------------------") var b = undefined;//沒有長度 console.log("isExitsParamsVariable(b)===" + isExitsParamsVariable(b)); console.log("isExitsVariable(b)===" + isExitsVariable(b)); console.log("--------------------------------------------------") var c = null;//沒有長度 console.log("isExitsParamsVariable(c)===" + isExitsParamsVariable(c)); console.log("isExitsVariable(c)===" + isExitsVariable(c)); console.log("--------------------------------------------------") var d = 0;//長度undefined console.log("isExitsParamsVariable(d)===" + isExitsParamsVariable(d)); console.log("isExitsVariable(d)===" + isExitsVariable(d)); console.log("--------------------------------------------------") var e = "";//長度為0 console.log("isExitsParamsVariable(e)====" + isExitsParamsVariable(e)); console.log("isExitsVariable(e)===" + isExitsVariable(e)); console.log("--------------------------------------------------") /*未定義聲明 f 則log會報錯：Uncaught ReferenceError: f is not defined ，不會執行兩個判斷方法*/ console.log("isExitsParamsVariable(f)====" + isExitsParamsVariable(f));//f:undefined console.log("isExitsVariable(f)===" + isExitsVariable(f));</script></body></html>希望本文所述對大家JavaScript程序設計有所幫助。
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