JavaScript TypeError – 无法访问“Y”的属性“X”

如果对未定义或空值执行属性访问，则会发生此 JavaScript 异常无法访问属性。

信息：

TypeError: Unable to get property {x} of undefined or 
           null reference (Edge)
TypeError: can't access property {x} of {y} (Firefox)
TypeError: {y} is undefined, can't access property {x} 
           of it (Firefox)
TypeError: {y} is null, can't access property {x} of it
           (Firefox)

例子：

TypeError: x is undefined, can't access property "prop" 
           of it 
TypeError: x is null, can't access property "prop" of it 
TypeError: can't access property "prop" of undefined
TypeError: can't access property "prop" of null

错误类型：

TypeError

错误原因：对代码中的任何未定义或空值执行了属性访问。

示例 1：在本示例中，GFG 未定义，因此发生错误。

HTML

    var GFG = undefined;
    GFG.substring(3); // error here

输出（在控制台中）：

TypeError: Unable to get property 'substring' of undefined or null reference

例2：在本例中，GFG为空，所以发生了错误。

HTML

    var GFG = null;
    GFG.substring(3); // error here

输出（在控制台中）：

TypeError: Unable to get property 'substring' of undefined or null reference