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JS為什麽出現undefined

js?產生undefined的情況:?

1.<script?type="text/javascript">

var?outObj?=?{?

type?:"java"

}

function?innerM(){

var?p;

alert(p);//undefined

alert("k:"+k);//error--innerObj未定義

alert(outObj.jack);//undefined

alert(innerObj.jack);//error--innerObj未定義

}

innerM();

</script>

2.?<script?type="text/javascript">

alert(typeof(false)?===?'boolean');?//true

alert(typeof(0)?===?'number');?//true

alert(typeof("")?===?'string');?//true

alert(typeof(null)?===?'object');?//true

alert(typeof?undefined?===?'undefined');?//true

</script>

3.?<script?type="text/javascript">

alert(false?==?undefined);?//false

alert(false?==?null);?//false?

alert(false?==?0);?//true

alert(false?==?"");?//true

alert(null?==?undefined);?//true

</script>

4.<script?type="text/javascript">

alert(false.toString());//?"false"

alert("".charAt(0));//?""

alert((0).toExponential(10));?//?0.0000000e+0

alert(undefined.toString());//?throw?exception?"undefined?has?no?properties"

alert(null.toString());?//?"null?has?no?properties"

</script>

5.<script?type="text/javascript">

alert(String(false));//?"false"

alert(String(""));//?""

alert(String(0));?//?0.0000000e+0

alert(String(undefined));//?"undefined"

alert(String(null));?//?"null"

alert(decodeURI(undefined));//?"undefined"

alert(decodeURI(null));//?"null"

</script>