{"id":2591,"date":"2021-07-12T10:05:08","date_gmt":"2021-07-12T02:05:08","guid":{"rendered":"http:\/\/ufqi.com\/blog\/?p=2591"},"modified":"2021-07-16T09:47:46","modified_gmt":"2021-07-16T01:47:46","slug":"javascript-undefined-null-detection","status":"publish","type":"post","link":"https:\/\/ufqi.com\/blog\/javascript-undefined-null-detection\/","title":{"rendered":"JavaScript\u672a\u5b9a\u4e49\u53d8\u91cf\u68c0\u6d4bundefined null detection"},"content":{"rendered":"

HTML5\u81ea\u5341\u591a\u5e74\u524d\uff082010\u5e74\u524d\u540e\uff09\u88ab\u63a8\u4e3e\u4ee5\u6765\uff0c\u9010\u6e10\u5927\u653e\u5f02\u5f69\uff0c\u5c3d\u7ba1\u8fc7\u53bb\u5341\u5e74\u662f\u5c5e\u4e8eNative APP\u7684\u9ad8\u5149\u65f6\u523b\uff0c\u4f46HTML5\u7684\u4e00\u4e9b\u65b0\u7279\u6027\u548c\u529f\u80fd\u8fd8\u662f\u5e26\u6765\u4e86\u5927\u91cf\u7684\u65b0\u7f51\u7edc\/\u7f51\u9875\u5e94\u7528\uff0c\u5c24\u5176\u662f\u5728JavaScript\u7684\u52a0\u6301\u4e0b\uff0c\u7f51\u9875\u5e94\u7528\u6709\u6781\u5176\u4ee4\u4eba\u7740\u8ff7\u7684\u4fbf\u5229\u4e0e\u5f3a\u5927\u3002<\/p>\n

JavaScipt\u662f\u7f51\u9875\u5e94\u7528\u9ed8\u8ba4\u7f16\u7a0b\u8bed\u8a00\uff0c\u8ba9\u4eba\u53c8\u7231\u53c8\u6068\u3002<\/p>\n

\u7b14\u8005\u66fe\u7ecf\u5728\u300a\u4e2d\u56fd\u8ba1\u7b97\u673a\u5b66\u4f1a\u901a\u8baf\u300b\u4e0a\u520a\u53d1\u4e86\u4e00\u7bc7\u770b\u6cd5\u6587\u7ae0\u300aJavaScript\u6216\u6210\u4e3b\u5bfc\u7684\u7f16\u7a0b\u8bed\u8a00<\/a>\u300b\uff08\u00a0https:\/\/dl.ccf.org.cn\/institude\/institudeDetail?id=3738875941521408&_ack=1 \uff09\uff0c\u5176\u540e\u4e24\u4e2a\u5f00\u6e90\u9879\u76ee GTAjax <\/a>(\u00a0https:\/\/ufqi.com\/dev\/gtajax )\u548c Hanjst <\/a>(\u00a0https:\/\/ufqi.com\/dev\/hanjst\/Hanjst.demo.html )\u53c8\u5206\u522b\u662f\u4f7f\u7528JavaScript\u505a\u4e86\u5b9e\u73b0\uff0c\u751a\u81f3 Base62x<\/a>(\u00a0https:\/\/ufqi.com\/dev\/base62x\/ ) \u7684\u7f16\u89e3\u7801\u65b9\u6cd5\uff0c\u90fd\u6709JavaScript\u8bed\u8a00\u7248\u672c\uff0c\u53ef\u8c13\u662fJavaScript\u7684\u91cd\u5ea6\u7528\u6237\u3001\u5f00\u53d1\u8005\u3002<\/p>\n

\u5373\u4fbf\u5982\u6b64\u201c\u8d44\u6df1\u201d\uff0c\u4ecd\u5e38\u5e38\u89c9\u5f97JavaScript\u7528\u8d77\u6765\u8ba9\u4eba\u4e0d\u90a3\u4e48\u653e\u5fc3\uff0c\u4e3a\u4e86\u5199\u51fa\u66f4\u53ef\u9760\u7684\u4ee3\u7801\uff0c\u6211\u4eec\u751a\u81f3\u5728JavaScript\u4e2d\u663e\u793a\u5730\u58f0\u660e\u7528\u4e25\u683c\u68c0\u67e5\u6a21\u5f0f: use strict.<\/p>\n

\u5373\u4fbf\u5982\u6b64\u201c\u8c28\u614e\u201d\uff0c\u6211\u4eec\u4e5f\u4f1a\u9047\u5230 object\/variable is null or undefined \u7684\u5f02\u5e38\u629b\u51fa\uff0c\u4f7f\u5f97\u7a0b\u5e8f\u5f02\u5e38\u7ec8\u6b62\uff0c\u4e3a\u7528\u6237\u5e26\u6765\u4e86\u5f88\u4e0d\u597d\u7684\u4f53\u9a8c\u3002\u968f\u7740\u6211\u4eec\u5728 UfqiWork\/\u6709\u798f\u5de5\u574a<\/a>(\u00a0https:\/\/ufqi.com\/work ) \u7684\u5f00\u53d1\u63a8\u8fdb\uff0c\u6211\u4eec\u5c1d\u8bd5\u5bfb\u627e\u4e00\u79cd\u68c0\u6d4b JavaScript \u53d8\u91cf\u6216\u8005\u5bf9\u8c61\u662f\u5426null \u6216 undefined\u65b9\u6cd5\u7684\u52aa\u529b\uff0c\u6709\u4e86\u65b0\u6536\u83b7\uff0c\u5179\u5206\u4eab\u5982\u4e0b\u3002<\/p>\n

\u4eceJavaScript\u672c\u8eab\u6765\u8bf4\uff0c\u8981\u68c0\u6d4b\u67d0\u4e2a\u5bf9\u8c61\u6216\u53d8\u91cf\u662f\u5426\u4e3anull\/undefined\uff0c\u4e3b\u8981\u6709\u4e24\u4e2a\u5de5\u5177\u53ef\u7528\uff1a typeof \u64cd\u4f5c\u7b26 \u548c window.hasOwnProperty \u65b9\u6cd5\u3002\u6211\u4eec\u6240\u521b\u5236\u7684\u65b9\u6cd5\uff0c \u4ee3\u78011\uff0c\u4e5f\u662f\u57fa\u4e8e\u8fd9\u4e24\u4e2a\u57fa\u672c\u64cd\u4f5c\u3002<\/p>\n

\n

\/\/- _isDefined
\/\/- @param: $var, to be tested;
\/\/- @param: global_or_local: ‘global|local’, optional
\/\/- return true | false
\/\/- usage: test whether a global variable($var): _isDefined(“$var”) , _isDefined($var)
\/\/- test a variable($var) declared nearby\/locally: _isDefined($var) , _isDefined(“$var”, “local”)
function _isDefined($var, global_or_local){
var isDef = false;
if($var != null){
var myTp = typeof $var;
if(myTp == ‘string’){
if(window.hasOwnProperty($var)){
isDef = true;
}
else if(global_or_local != null && global_or_local == ‘local’){
isDef = true;
}
else{
\/\/- @todo
}
}
else if(myTp != ‘undefined’){
isDef = true;
}
}
return isDef;
}<\/p>\n

\n

\u8fd9\u4e9b\u4ee3\u7801\u5df2\u7ecf\u90e8\u7f72\u5728 Hanjst\/\u6c49\u5409\u65af\u7279\uff0c UfqiWork\/\u6709\u798f\u5de5\u574a \u76f8\u5173\u6a21\u5757\u4e2d\uff0c\u7ecf\u5b9e\u6d4b\uff0c\u8fd0\u884c\u826f\u597d\uff0c\u7b26\u5408\u9884\u671f\u3002<\/p>\n

\u5728\u7f16\u5236\u8fd9\u4e9b\u4ee3\u7801\u4e4b\u524d\uff0c\u6211\u4eec\u4e5f\u66fe\u7ecf\u5728\u591a\u4e2a\u5e94\u7528\u4e2d\u5199\u4e86\u5927\u91cf\u7684\u68c0\u6d4b\u67d0\u4e2aJavaScript\u662f\u5426\u4e3anull\/undefined\uff0c\u65e0\u4e00\u4f8b\u5916\u5730\u90fd\u662f\u4f7f\u7528 typeof \u6216\u8005 window.hasOwnProperty\u65b9\u6cd5\uff0c\u751a\u81f3\u518d\u5e26\u7740\u5bf9 null\u548c empty\u7684\u5224\u65ad\u3002\u4ee3\u7801\u663e\u5f97\u5f02\u5e38\u7e41\u7410\u548c\u81c3\u80bf\u3002\u76f8\u4fe1\u6b64\u540e\uff0c\u8fd9\u4e9b\u4ee3\u7801\u5c06\u9646\u7eed\u5f97\u5230\u7b80\u5316\u548c\u4fee\u6b63\uff0c\u65b0\u5199\u7684\u4ee3\u7801\u4e5f\u5c06\u5f15\u5165\u8be5\u529f\u80fd _isDefined .<\/p>\n

\u5728\u7f16\u5236\u8fd9\u4e9b\u4ee3\u7801\u4e4b\u524d\uff0c\u6211\u4eec\u4e5f\u5728\u7f51\u7edc\u4e0a\u8fdb\u884c\u4e86 code review\uff0c\u7ffb\u9605\u4e86\u5927\u91cf\u9488\u5bf9\u6b64\u95ee\u9898\u7684\u89e3\u51b3\u601d\u8def\u548c\u529e\u6cd5\uff0c\u53ef\u80fd\u9762\u5bf9\u4e0d\u540c\u8fd0\u884c\u65f6\u73af\u5883\uff0c\u7ed9\u51fa\u7684\u89e3\u51b3\u65b9\u6cd5\u4e5f\u5404\u4e2a\u4e0d\u540c\uff0c\u5b9e\u9645\u4e0a\u5e76\u6ca1\u6709\u4e00\u79cd\u65b9\u6cd5\u53ef\u4ee5\u901a\u7528\u5230\u201c\u653e\u4e4b\u56db\u6d77\u800c\u7686\u51c6\u201d\u3002<\/p>\n

\u6bd4\u5982\u6700\u63a5\u8fd1\u672c\u65b9\u6cd5\u7684\u4e00\u4e2a\u5c1d\u8bd5\u662f\u5b9a\u4e49\u4e00\u4e2a isDefined2, \u4ee3\u78012\uff0c\u7136\u540e\u5728\u65b9\u6cd5\u4f53\u5185\u4f7f\u7528 typeof \u64cd\u4f5c\u7b26\u5224\u65ad\u4f20\u5165\u7684 $var \u662f\u5426 null\/undefined , \u4ee5\u6b64\u6765\u8fdb\u884c\u5224\u65ad\u53d8\u91cf\u6216\u8005\u5bf9\u8c61 $var \u662f\u5426\u4e3a null\/undefined .<\/p>\n

\n

\/\/- _isDefined2,\u00a0 WRONG!
\/\/- @param: $var, to be tested
\/\/- return true | false
function _isDefined2($var){
var isDef = false;
if(typeof $var != ‘undefined’){
isDef = true;
}
return isDef;
}<\/p>\n

\n

\u8fd9\u6bb5\u4ee3\u7801\u901a\u5e38\u60c5\u51b5\u4e0b\uff0c\u80fd\u591f\u8fdb\u884c\u8fd0\u884c\uff0c\u5e76\u5bf9\u4e00\u4e9b\u53d8\u91cf\u5b9e\u73b0null\/undefined\u7684\u63a2\u6d4b\u3002\u7136\u800c\u8fd9\u4e2a\u60c5\u51b5\u53ea\u5c40\u9650\u5728\u672c\u5730\u53d8\u91cf\u6216\u8005\u5f53\u524d\u5df2\u7ecf\u58f0\u660e\u7684\u53d8\u91cf\uff0c\u5982\u679c\u5f85\u63a2\u6d4b\u7684\u53d8\u91cf $var \u6ca1\u6709\u5728 _isDefined2 \u8c03\u7528\u4e4b\u524d\u88ab\u58f0\u660e\uff0c\u5219\u8c03\u7528\u63a2\u6d4b\u8bed\u53e5 _isDedined2($var) \u672c\u8eab\u5c31\u4f1a\u62a5\u9519\u8bf4 $var is null\/undefined .<\/p>\n

\u95ee\u9898\u8fdb\u4e00\u6b65\u5730\u88ab\u5ef6\u7533\u5230 JavaScript\u7684\u51fd\u6570\u65b9\u6cd5\u8c03\u7528\u7684\u53c2\u6570\u4f20\u9012\u662fpass-by-value\u4f20\u503c\uff0c\u8fd8\u662f pass-by-reference\u4f20\u5f15\u7528\u3002\u901a\u5e38\u60c5\u51b5\u4e0b\uff0cJavaScript\u7684\u65b9\u6cd5\u8c03\u7528\u90fd\u662f pass-by-value, \u6240\u4ee5\u5728\u6267\u884c\u8bed\u53e5\u00a0 _isDefined($var) \u65f6\uff0c\u5148\u8981\u5bf9 $var \u8fdb\u884c\u53d6\u503c\u64cd\u4f5c\uff0c\u800c $var \u5982\u679c\u672c\u8eab undefined \u65f6\uff0c\u60b2\u5267\u9519\u8bef\u5c31\u4ea7\u751f\u4e86\uff0c\u68c0\u6d4b\u662f\u5426\u4e3a null\/undefined \u4e4b\u524d\u5df2\u7ecf\u53d1\u751f\u4e86 undefined .<\/p>\n

\u5f53\u6211\u4eec\u5224\u65ad\u4e86 $var \u4e3aString\u7c7b\u578b\u7684\u6570\u636e\u5bf9\u8c61\u65f6\uff0c\u540c\u65f6\u5f15\u53d1\u4e86\u53e6\u5916\u4e00\u4e2a\u95ee\u9898\uff0c\u5c31\u662f\u5f53\u67d0\u4e00\u4e2a\u6570\u636e\u53d8\u91cf\u6216\u5bf9\u8c61\u5176\u672c\u8eab\u5c31\u662f\u5b57\u7b26\u4e32\u65f6\uff0c\u5982\u679c\u5176\u672c\u8eab\u662f\u4e00\u4e2a\u672c\u5730\u53d8\u91cf\uff0c\u800cString\u7c7b\u578b\u4f1a\u88ab\u89c6\u4e3a\u68c0\u6d4b\u5168\u5c40\u53d8\u91cf\uff0c\u8fd9\u91cc\u5c31\u4f1a\u53d1\u751f\u5f02\u5e38\u9519\u8bef\uff0c\u628a\u672c\u8be5\u8fd4\u56de\u672c\u5730\u53d8\u91cf\u4e3a true\u7684\u7ed3\u679c\uff0c\u9519\u8bef\u5730\u89e3\u91ca\u4e3a\u5168\u5c40\u53d8\u91cf\u4e3a false\u3002\u56e0\u6b64\u9700\u8981\u8fdb\u4e00\u6b65\u5730\u5bf9\u65b9\u6cd5\u672c\u8eab\u505a\u4fee\u6b63\uff0c\u4f7f\u4e4b\u80fd\u591f\u517c\u5bb9\u672c\u5730\u53d8\u91cf\u3001String\u7c7b\u578b\uff0c\u6539\u8fdb\u7684\u505a\u6cd5\u662f\u9488\u5bf9 _isDefined \u5f15\u5165\u7b2c\u4e8c\u4e2a\u53c2\u6570\uff0c\u663e\u793a\u5730\u544a\u8bc9\u7a0b\u5e8f\uff0c\u5f53\u524d\u867d\u7136\u5f15\u5165\u4e86 String\u7c7b\u578b\u7684\u53d8\u91cf\uff0c\u4f46\u4e0d\u662f\u9ed8\u8ba4\u68c0\u6d4b\u5168\u5c40\u53d8\u91cf\uff0c\u800c\u662f\u8981\u68c0\u6d4b\u672c\u5730\u53d8\u91cf\u3002<\/p>\n

\u8fdb\u4e00\u6b65\u5730\u4fee\u6b63\u8fd9\u4e2a\u95ee\u9898\uff0c\u6211\u4eec\u521b\u5236\u4e86\u6837\u4f8b\u4ee3\u78011. \u53ef\u4ee5\u517c\u5bb9\u5168\u5c40\u53d8\u91cf\u548c\u672c\u5730\u53d8\u91cf\uff0c\u4e0d\u7ba1$var \u662f\u5426\u88ab\u4ec0\u4e48\uff0c\u90fd\u80fd\u591f\u8fdb\u884c\u68c0\u6d4b\u3002\u53ea\u662f\uff0c\u5728\u8c03\u7528\u7684\u65f6\u5019\u9700\u8981\u533a\u5206\uff0c\u5f85\u68c0\u6d4b\u7684\u53d8\u91cf\u6216\u8005\u5bf9\u8c61\u662f\u5168\u5c40\u53d8\u91cf\u6216\u8005\u662f\u5c40\u90e8\u53d8\u91cf\uff1a
\u5982\u679c\u662f\u5168\u5c40\u53d8\u91cf\uff0c\u8c03\u7528\u65b9\u5f0f\u662f\uff1a _isDefined(“$var”) ,\u00a0_isDefined($var)
\u5982\u679c\u662f\u5c40\u90e8\u53d8\u91cf\uff0c\u8c03\u7528\u65b9\u6cd5\u662f\uff1a _isDefined($var, ‘local’) ,\u00a0_isDefined(“$var”, ‘local’)\u00a0<\/p>\n

\u53d8\u91cf $var \u5728\u88ab\u68c0\u6d4b\u4e4b\u524d\uff0c\u6211\u4eec\u80fd\u591f\u77e5\u9053\u5b83\u662f\u5168\u5c40\u53d8\u91cf\u6216\u8005\u5c40\u90e8\u53d8\u91cf\u5417\uff1f
\u7b54\u6848\u5e94\u8be5\u662f\u660e\u786e\u7684\u3002<\/p>\n

\u6d4b\u8bd5\u6837\u4f8b\uff1a<\/p>\n

var a;
console.log(“UfqiWork\/Hanjst: a:”+_isDefined(a));
var b = null;
console.log(“UfqiWork\/Hanjst: b:”+_isDefined(b));
var c = 0;
console.log(“UfqiWork\/Hanjst: c:”+_isDefined(c));
console.log(“UfqiWork\/Hanjst: global-c:”+_isDefined(“c”));
var dd = “d_string”;
console.log(“UfqiWork\/Hanjst: local-dd:”+_isDefined(dd, ‘local’));
console.log(“UfqiWork\/Hanjst: global-dd:”+_isDefined(dd));
console.log(“UfqiWork\/Hanjst: global-dd:”+_isDefined(“dd”));
console.log(“UfqiWork\/Hanjst: local-dd:”+_isDefined(“dd”, ‘local’));<\/p>\n

\u9884\u671f\u8f93\u51fa\uff1a<\/p>\n

UfqiWork\/Hanjst: a:false
UfqiWork\/Hanjst: b:false
UfqiWork\/Hanjst: c:true
UfqiWork\/Hanjst: global-c:false
UfqiWork\/Hanjst: local-dd:true
UfqiWork\/Hanjst: global-dd:false
UfqiWork\/Hanjst: global-dd:false
UfqiWork\/Hanjst: local-dd:true<\/p>\n

\n

\"Hanjst\"
Hanjst \u6c49\u5409\u65af\u7279 Logo<\/p>\n

\ud83d\ude4b<\/span>Hanjst\u6c49\u5409\u65af\u7279 \u662f\u4e00\u79cd\u57fa\u4e8eJavaScript\u7684\u6a21\u677f\u8bed\u8a00\u53ca\u6a21\u7248\u89e3\u6790\u5f15\u64ce\uff0c\u5979\u8fd0\u884c\u5728\u5ba2\u6237\u7aef\u6216\u670d\u52a1\u5668\u7aef\u3002<\/p>\n

\ud83d\ude4bHanjst\u6c49\u5409\u65af\u7279 \u80fd\u591f\u8868\u8ff0\u903b\u8f91\u63a7\u5236\uff0c\u80fd\u591f\u5b9e\u73b0\u4e0e\u670d\u52a1\u5668\u7aef\u6a21\u7248\u8bed\u8a00\u76f8\u540c\u7684\u5f3a\u5927\u529f\u80fd\u3002<\/p>\n