JavaScript 如何获取对象的第一个键名

给定一个对象，任务是获取JavaScript对象的第一个键名。由于JavaScript对象不包含有序索引，因此我们可以使用以下方法来获取对象的第一个键名。

方法1

首先将JavaScript对象存储在一个变量中。
使用 object.keys(objectName) 方法来访问对象的所有键名。
现在，我们可以使用类似 Object.keys(objectName)[0] 的索引来获取对象的第一个元素的键名。

示例： 这个示例说明了上述的方法。

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        How to get the first key name 
        of a JavaScript object ? 
    </title>     
</head> 
  
<body style = "text-align:center;"> 
      
    <h1 style = "color:green;" > 
        GeeksforGeeks 
    </h1> 
      
    <p id = "GFG_UP1" style = 
        "font-size: 15px; font-weight: bold;"> 
    </p> 
      
    <p id = "GFG_UP2" style = 
        "font-size: 15px; font-weight: bold; color: green;"> 
    </p> 
      
    <button onclick = "GFG_Fun()"> 
        click here 
    </button> 
      
    <p id = "GFG_DOWN" style = 
        "color:green; font-size: 20px; font-weight: bold;"> 
    </p> 
      
    <script> 
        var up1 = document.getElementById('GFG_UP1'); 
        var up2 = document.getElementById('GFG_UP2'); 
        var down = document.getElementById('GFG_DOWN'); 
          
        var obj = { 
            "Prop_1": ["Val_11", "Val_12", "Val_13"], 
            "Prop_2": ["Val_21", "Val_22", "Val_23"], 
            "Prop_3": ["Val_31", "Val_32", "Val_33"] 
        }; 
          
        up1.innerHTML = "Click on the button to get the "+ 
                                "first key of Object."; 
          
        up2.innerHTML = JSON.stringify(obj); 
          
        function GFG_Fun() { 
            down.innerHTML = "The first key = '" + 
            Object.keys(obj)[0] + "' <br> Value = '" 
            + obj[Object.keys(obj)[0]] + "'"; 
        } 
    </script> 
</body> 
  
</html>

输出：

JavaScript 如何获取对象的第一个键名

方法2

首先，将JavaScript对象存储在一个变量中。
通过循环的方式，开始访问JavaScript对象的所有键值。
运行一次循环后，跳出循环。然后我们就可以得到对象的第一个键值。

示例： 此示例演示了上述方法。

<!DOCTYPE HTML> 
<html> 
  
<head> 
    <title> 
        How to get the first key name 
        of a JavaScript object ? 
    </title>     
</head> 
  
<body style = "text-align:center;"> 
      
    <h1 style = "color:green;" > 
        GeeksforGeeks 
    </h1> 
      
    <p id = "GFG_UP1" style = 
        "font-size: 15px; font-weight: bold;"> 
    </p> 
      
    <p id = "GFG_UP2" style = 
        "font-size: 15px; font-weight: bold; color: green;"> 
    </p> 
      
    <button onclick = "GFG_Fun()"> 
        click here 
    </button> 
      
    <p id = "GFG_DOWN" style = 
        "color:green; font-size: 20px; font-weight: bold;"> 
    </p> 
      
    <script> 
        var up1 = document.getElementById('GFG_UP1'); 
        var up2 = document.getElementById('GFG_UP2'); 
        var down = document.getElementById('GFG_DOWN'); 
          
        var obj = { 
            "Prop_1": ["Val_11", "Val_12", "Val_13"], 
            "Prop_2": ["Val_21", "Val_22", "Val_23"], 
            "Prop_3": ["Val_31", "Val_32", "Val_33"] 
        }; 
          
        up1.innerHTML = "Click on the button to get " 
                        + "the first key of Object."; 
          
        up2.innerHTML = JSON.stringify(obj); 
          
        function GFG_Fun() { 
            var key; 
              
            for (var k in obj) { 
                key = k; 
                break; 
            } 
              
            down.innerHTML = "The first key = '" + key + 
                    "' <br> Value = '" + obj[key] + "'"; 
        } 
    </script> 
</body> 
  
</html>

输出：

JavaScript 如何获取对象的第一个键名