数组长度扩充后默认填充的是null，不是undefined

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#1 Created at ... [Delete] [Delete and Lock User]

var arr1=[1,2,3,4];

arr1[7]='五';

var x=JSON.stringify(arr1);

console.log(x);//[1,2,3,4,null,null,null,"五"],not undefined

#2 Created at ... [Delete] [Delete and Lock User]

console.log(JSON.stringify( [ undefined ] ))

'[null]'

#3 Created at ... [Delete] [Delete and Lock User]

明白了，谢谢老师