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int,uint - 0
Boolean - false
Number - NaN
String,Array,Object - null
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var a1;
var a2 = true;
var a3 = 1;
var a4 = "Hello";
var a5 = new Object();
var a6 = null;
var a7 = NaN;
var a8 = undefined;
alert(typeof a); //æ¾ç¤º"undefined"
alert(typeof a1); //æ¾ç¤º"undefined"
alert(typeof a2); //æ¾ç¤º"boolean"
alert(typeof a3); //æ¾ç¤º"number"
alert(typeof a4); //æ¾ç¤º"string"
alert(typeof a5); //æ¾ç¤º"object"
alert(typeof a6); //æ¾ç¤º"object"
alert(typeof a7); //æ¾ç¤º"number"
alert(typeof a8); //æ¾ç¤º"undefined"
ä»ä¸é¢ç代ç ä¸å¯ä»¥çåºæªå®ä¹çå¼åå®ä¹æªèµå¼ç为 undefinedï¼nullæ¯ä¸ç§ç¹æ®çobject,NaNæ¯ä¸ç§ç¹æ®çnumberã
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var a1; //a1çå¼ä¸ºundefined
var a2 = null;
var a3 = NaN;
alert(a1 == a2); //æ¾ç¤º"true"
alert(a1 != a2); //æ¾ç¤º"false"
alert(a1 == a3); //æ¾ç¤º"false"
alert(a1 != a3); //æ¾ç¤º"true"
alert(a2 == a3); //æ¾ç¤º"false"
alert(a2 != a3); //æ¾ç¤º"true"
alert(a3 == a3); //æ¾ç¤º"false"
alert(a3 != a3); //æ¾ç¤º"true"
ä»ä¸é¢ç代ç å¯ä»¥å¾åºç»è®ºï¼ï¼1ï¼undefinedä¸nullæ¯ç¸çï¼ ï¼2ï¼NaNä¸ä»»ä½å¼é½ä¸ç¸çï¼ä¸èªå·±ä¹ä¸ç¸çã
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<script type="text/javascript">
var t1=""
var t2
if (t1===undefined) {document.write("t1 is undefined")}
if (t2===undefined) {document.write("t2 is undefined")}
</script>
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t2 is undefined
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请注æï¼å¨ Jscript ä¸ï¼null ä¸ 0 ä¸ç¸çï¼ä¸å¨ C å C++ ä¸ä¸åï¼ãåæ¶åºè¯¥æåºçæ¯ï¼Jscriptä¸ typeof è¿ç®ç¬¦å°æ¥å null å¼ä¸º Object ç±»åï¼èéç±»å nullãè¿ç¹æ½å¨çæ··æ·æ¯ä¸ºäºåä¸å
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注æä¸è½éè¿ä¸ undefined åæ¯è¾æ¥æµè¯ä¸ä¸ªåéæ¯å¦åå¨ï¼è½ç¶å¯ä»¥æ£æ¥å®çç±»åæ¯å¦ä¸ºâundefinedâãå¨ä»¥ä¸ç代ç èä¾ä¸ï¼å设ç¨åºåæ³æµè¯æ¯å¦å·²ç»å£°æåé x ï¼
// è¿ç§æ¹æ³ä¸èµ·ä½ç¨
if (x == undefined)
// ä½æäºæä½
// è¿ä¸ªæ¹æ³åæ ·ä¸èµ·ä½ç¨- å¿
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// å符串 "undefined"
if (typeof(x) == undefined)
// ä½æäºæä½
// è¿ä¸ªæ¹æ³ææ
if (typeof(x) == "undefined")
// ä½æäºæä½
èèå° undefined å¼ä¸nullåæ¯è¾ã
someObject.prop == null;
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åµæ¶ï¼æ¯è¾çç»æ为 trueï¼
å¦æå±æ§ someObject.prop å
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å¦æå±æ§ someObject.prop ä¸åå¨ã
è¦æ£æ¥ä¸ä¸ªå¯¹è±¡å±æ§æ¯å¦åå¨ï¼å¯ä»¥ä½¿ç¨æ°ç in è¿ç®ç¬¦ï¼
if ("prop" in someObject)
// someObject æå±æ§ 'prop'
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- nullæ¯å¯¹è±¡(空对象, 没æä»»ä½å±æ§åæ¹æ³)ï¼undefinedæ¯undefinedç±»åçå¼ãè¯è¯ä¸é¢ç代ç ï¼
document.writeln(typeof null); //return object
document.writeln(typeof undefined); //return undefined
- 对象模åä¸ï¼ææç对象é½æ¯Objectæå
¶åç±»çå®ä¾ï¼ä½null对象ä¾å¤ï¼
document.writeln(null instanceof Object); //return false
- nullâçå¼(==)âäºundefinedï¼ä½ä¸âå
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document.writeln(null == undefined); //return true
document.writeln(null === undefined); //return false
- è¿ç®æ¶nullä¸undefinedé½å¯ä»¥è¢«ç±»å转æ¢ä¸ºfalseï¼ä½ä¸çå¼äºfalseï¼
document.writeln(!null, !undefined); //return true,true
document.writeln(null==false); //return false
document.writeln(undefined==false); //return false
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