在此页

• Message
• Error type
• What went wrong?
• 范例
• 另请参阅
• 相关话题

The JavaScript exception "can't access property" occurs when property access was operated on undefined or null values.

Message

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)
范例：
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
					

Error type

TypeError .

What went wrong?

The property access was operated on undefined or null 值。

范例

Invalid cases

// undefined and null cases on which the substring method won't work
var foo = undefined;
foo.substring(1); // TypeError: x is undefined, can't access property "substring" of it
var foo = null;
foo.substring(1); // TypeError: x is null, can't access property "substring" of it
					

Fixing the issue

To fix null pointer to undefined or null values, you can use the typeof operator, for example.

if (typeof foo !== 'undefined') {
  // Now we know that foo is defined, we are good to go.
}
					

另请参阅

• undefined
• null

元数据

• 最后修改： Apr 16, 2020

相关话题

1. JavaScript
2. 教程：
3. 完全初学者

   1. JavaScript 基础
   2. JavaScript 第一步
   3. JavaScript 构建块
   4. 引入 JavaScript 对象
4. JavaScript 指南

   1. 介绍
   2. 语法和类型
   3. 控制流程和错误处理
   4. 循环和迭代
   5. 函数
   6. 表达式和运算符
   7. 数字和日期
   8. 文本格式化
   9. 正则表达式
   10. 索引集合
   11. 键控集合
   12. Working with objects
   13. 对象模型的细节
   14. Using promises
   15. 迭代器和生成器
   16. Meta programming
   17. JavaScript 模块
5. 中间体

   1. 引入 JavaScript 对象
   2. 客户端侧 Web API
   3. 重新介绍 JavaScript
   4. JavaScript 数据结构
   5. 相等比较和相同
   6. 闭包
6. 高级

   1. 继承和原型链
   2. 严格模式
   3. JavaScript 类型数组
   4. 内存管理
   5. 并发模型和事件循环
7. 参考：
8. 内置对象

   1. AggregateError
   2. Array
   3. ArrayBuffer
   4. AsyncFunction
   5. AsyncIterator
   6. Atomics
   7. BigInt
   8. BigInt64Array
   9. BigUint64Array
   10. 布尔
   11. DataView
   12. Date
   13. Error
   14. EvalError
   15. Float32Array
   16. Float64Array
   17. Function
   18. Generator
   19. GeneratorFunction
   20. Infinity
   21. Int16Array
   22. Int32Array
   23. Int8Array
   24. InternalError
   25. Intl
   26. Intl.Collator
   27. Intl.DateTimeFormat
   28. Intl.DisplayNames
   29. Intl.ListFormat
   30. Intl.Locale
   31. Intl.NumberFormat
   32. Intl.PluralRules
   33. Intl.RelativeTimeFormat
   34. Iterator
   35. JSON
   36. Map
   37. Math
   38. NaN
   39. Number
   40. Object
   41. Promise
   42. Proxy
   43. RangeError
   44. ReferenceError
   45. Reflect
   46. RegExp
   47. Set
   48. SharedArrayBuffer
   49. String
   50. Symbol
   51. SyntaxError
   52. TypeError
   53. TypedArray
   54. URIError
   55. Uint16Array
   56. Uint32Array
   57. Uint8Array
   58. Uint8ClampedArray
   59. WeakMap
   60. WeakSet
   61. WebAssembly
   62. decodeURI()
   63. decodeURIComponent()
   64. encodeURI()
   65. encodeURIComponent()
   66. escape()
   67. eval()
   68. globalThis
   69. isFinite()
   70. isNaN()
   71. null
   72. parseFloat()
   73. parseInt()
   74. undefined
   75. unescape()
   76. uneval()
9. 表达式 & 运算符

   1. 算术运算符
   2. Array comprehensions
   3. 赋值运算符
   4. Bitwise operators
   5. 逗号运算符
   6. Comparison operators
   7. 条件 (三元) 运算符
   8. Destructuring assignment
   9. Expression closures
   10. 函数表达式
   11. Generator comprehensions
   12. Grouping operator
   13. Legacy generator function expression
   14. Logical operators
   15. Nullish coalescing operator
   16. 对象初始化器
   17. 运算符优先级
   18. Optional chaining
   19. Pipeline operator
   20. 特性访问器
   21. 传播句法
   22. 异步函数表达式
   23. await
   24. class expression
   25. delete operator
   26. function* 表达式
   27. in operator
   28. instanceof
   29. new operator
   30. new.target
   31. super
   32. this
   33. typeof
   34. void 运算符
   35. yield
   36. yield*
10. 语句 & 声明

    1. Legacy generator function
    2. async function
    3. block
    4. break
    5. class
    6. const
    7. continue
    8. debugger
    9. default
    10. do...while
    11. empty
    12. export
    13. for
    14. for await...of
    15. for each...in
    16. for...in
    17. for...of
    18. 函数声明
    19. function*
    20. if...else
    21. import
    22. import.meta
    23. label
    24. let
    25. return
    26. switch
    27. throw
    28. try...catch
    29. var
    30. while
    31. with
11. 函数

    1. 箭头函数表达式
    2. 默认参数
    3. 方法定义
    4. 其余参数
    5. 自变量对象
    6. getter
    7. setter
12. 类

    1. Class fields
    2. 构造函数
    3. extends
    4. static
13. 错误

    1. Error: Permission denied to access property "x"
    2. InternalError: too much recursion
    3. RangeError: argument is not a valid code point
    4. RangeError: invalid array length
    5. RangeError: invalid date
    6. RangeError: precision is out of range
    7. RangeError: radix must be an integer
    8. RangeError: repeat count must be less than infinity
    9. RangeError: repeat count must be non-negative
    10. ReferenceError: "x" is not defined
    11. ReferenceError: assignment to undeclared variable "x"
    12. ReferenceError: can't access lexical declaration`X' before initialization
    13. ReferenceError: deprecated caller or arguments usage
    14. ReferenceError: invalid assignment left-hand side
    15. ReferenceError: reference to undefined property "x"
    16. SyntaxError: "0"-prefixed octal literals and octal escape seq. are deprecated
    17. SyntaxError: "use strict" not allowed in function with non-simple parameters
    18. SyntaxError: "x" is a reserved identifier
    19. SyntaxError: JSON.parse: bad parsing
    20. SyntaxError: Malformed formal parameter
    21. SyntaxError: Unexpected token
    22. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    23. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    24. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
    25. SyntaxError: for-in loop head declarations may not have initializers
    26. SyntaxError: function statement requires a name
    27. SyntaxError: identifier starts immediately after numeric literal
    28. SyntaxError: illegal character
    29. SyntaxError: invalid regular expression flag "x"
    30. SyntaxError: missing ) after argument list
    31. SyntaxError: missing ) after condition
    32. SyntaxError: missing : after property id
    33. SyntaxError: missing ; before statement
    34. SyntaxError: missing = in const declaration
    35. SyntaxError: missing ] after element list
    36. SyntaxError: missing formal parameter
    37. SyntaxError: missing name after . operator
    38. SyntaxError: missing variable name
    39. SyntaxError: missing } after function body
    40. SyntaxError: missing } after property list
    41. SyntaxError: redeclaration of formal parameter "x"
    42. SyntaxError: return not in function
    43. SyntaxError: test for equality (==) mistyped as assignment (=)?
    44. SyntaxError: unterminated string literal
    45. TypeError: "x" has no properties
    46. TypeError: "x" is (not) "y"
    47. TypeError: "x" is not a constructor
    48. TypeError: "x" is not a function
    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
    63. TypeError: invalid 'instanceof' operand 'x'
    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
    73. Warning: Date.prototype.toLocaleFormat is deprecated
    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
14. 杂项

    1. JavaScript 技术概述
    2. 词法语法
    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. 过渡到严格模式
    8. 模板文字
    9. 弃用特征