JavaScript中JSON转数组遇到Uncaught TypeError: Cannot set properties of undefined (setting ‘变量名‘)

报错代码：

		    let list =${sessionScope.cateLogList};
			//json转数组
			let arr = [];
			for (var i in list) {
				arr[i]['catelogId'] = list[i].catelogId;
				arr[i]['catelogName'] = list[i].catelogName;
				console.log(arr[i]['catelogName']);
			}

定义了一个数组，想将接收到的Json转为数组，但是遇到了Uncaught TypeError报错

报错原因

一开始以为是错在list上，排查后发现是因为数组没有提前定义

解决方案

提前给数组进行赋null值

		let list =${sessionScope.cateLogList};
		//json转数组
		//提前对数组进行赋值
		let arr = [{
			'catelogId':null,
			'catelogName':null
		}];
		for ( var i = 0; i<list.length;i++){
			arr.push({
			'catelogId':null,
			'catelogName':null
			});
		}
		
		for (var i in list) {
			arr[i]['catelogId'] = list[i].catelogId;
			arr[i]['catelogName'] = list[i].catelogName;
			console.log(arr[i]['catelogName']);
		}

问题解决