有关JSON.stringify( )的问题

Question

letknowledgeData=[];letinitialKnowledge=[[["id",1],["title","关于float的那些事儿"],["URL","http://www.w3school.com.cn/cssref/pr_class_float.asp"],["progress",100],["evaluation",3],["notes","关于float的那些事儿关于float的那些事儿关于float的那些事儿关于float的那些事儿关于float的那些事儿关于float的那些事儿"],["tags",["1","2","3"]]],[["id",2],["title","position知多少"],["URL","http://www.w3school.com.cn/cssref/pr_class_position.asp"],["progress",75],["evaluation",4],["notes","position知多少position知多少position知多少position知多少position知多少position知多少position知多少position知多少position知多少"],["tags",["1","2"]]],[["id",3],["title","1211gfdhfnndhdfhdfgfd"],["URL","http://www.w3school.com.cn/cssref/pr_class_position.asp"],["progress",66],["evaluation",2],["notes","gfgitiongdfgfdh42356576654435u6uyuydhfjyrdjfjyrdhhvjtdyrdhhmvjdhhmvhthhgchvjyhmhvjyhdgmhvhyedtjfhjyyedtjykkfuuysjx"],["tags",["2","3"]]]];for(letknowledgeofinitialKnowledge){knowledgeData.push(newMap(knowledge));}localStorage.setItem("knowledgeData",JSON.stringify(knowledgeData));遇到问题：现在将knowledgeData这个数组转换成JSON字符串存到localStorage中，再使用JSON.parse(localStorage.getItem("knowledgeData"))取出，是空的，不知道为什么？是因为使用了Map的原因吗？求大神解答