Question

I'm using a callback to create an object, which I'd like to parse. I'm able to parse the object when the name/value pair I've specified exists, but unable to identify when the name/value pair in my object is undefined.

JSON Object with name/value pair populated:

parseMe([{"item1" : "", "item2" : "", "item3" : [], "item4" : "content goes here"}]);

JSON Object with name/value parsed undefined:

parseMe([{"error" : "Not available"}]);

Parsing logic EX:

    var renderR="";
    function parseMe(data){
     renderR="";
    if(data[0].item4!="collapse") renderR=data[0].item4; 
//if name/value pair isn't equal to "collapse" render it.
    else if(data[0].item4==='undefined'){ 
    document.getElementById('div1').style.display='none'; 
//if name/value pair is undefined in object hide the div.
     }
    }

Answer 1

您应该使用 typeof 来检查未定义的密钥。

   function parseMe(data){

        renderR="";

        if(data[0].item4!="collapse"){

            renderR=data[0].item4; 
            //if name/value pair isn't equal to "collapse" render it.

        }else if(typeof data[0].item4==="undefined"){ 

            document.getElementById('div1').style.display='none'; 
            //if name/value pair is undefined in object hide the div.

        } 
}

Answer 2

如上面的评论中所述，else if条件可以更改为typeof(...) === 'undefined'以使该方法有效。然而，更标准的解决方案是使用hasOwnProperty方法查询对象的键值，例如

} else if (data[0].hasOwnProperty('item4')) {

在尝试通过点符号或方括号访问属性之前，应首先执行此检查。此外，这不会遍历原型链以确定继承的属性，但它将检测直接属于该对象的任何属性，这对于原始对象值和解析逻辑（例如此）应该足够。因此，包含代码重构的最终解决方案可以使用hasOwnProperty方法，也可以使用in运算符和switch语句，例如

for (var key in data[0]) {
    var value = data[0][key];
    switch (key) {
        case "item1":
            dosomething(value);
            break;
        case "item2":
            dosomethingelse(value);
            break;
        // etc...
    }
}

Javascript parsing - recognizing undefined object name/value pairs

2 个答案: