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

JavaScript 严格模式 -only exception "applying the 'delete' operator to an unqualified name is deprecated" occurs when variables are attempted to be deleted using the delete operator.

Message

SyntaxError: Calling delete on expression not allowed in strict mode (Edge)
SyntaxError: applying the 'delete' operator to an unqualified name is deprecated (Firefox)
SyntaxError: Delete of an unqualified identifier in strict mode. (Chrome)
					

Error type

SyntaxError in 严格模式 only.

What went wrong?

Normal variables in JavaScript can't be deleted using the delete operator. In strict mode, an attempt to delete a variable will throw an error and is not allowed.

delete operator can only delete properties on an object. Object properties are "qualified" if they are configurable.

Unlike what common belief suggests, the delete operator has nothing to do with directly freeing memory. Memory management is done indirectly via breaking references, see the memory management page and the delete operator page for more details.

This error only happens in strict mode code . In non-strict code, the operation just returns false .

范例

Freeing the contents of a variable

Attempting to delete a plain variable, doesn't work in JavaScript and it throws an error in strict mode:

'use strict';
var x;
// ...
delete x;
// SyntaxError: applying the 'delete' operator to an unqualified name
// is deprecated
					

To free the contents of a variable, you can set it to null :

'use strict';
var x;
// ...
x = null;
// x can be garbage collected
					

另请参阅

• delete
• Memory management
• TypeError: property "x" is non-configurable and can't be deleted

元数据

• 最后修改： May 6, 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. Atomics
   6. BigInt
   7. BigInt64Array
   8. BigUint64Array
   9. 布尔
   10. DataView
   11. Date
   12. Error
   13. EvalError
   14. Float32Array
   15. Float64Array
   16. Function
   17. Generator
   18. GeneratorFunction
   19. Infinity
   20. Int16Array
   21. Int32Array
   22. Int8Array
   23. InternalError
   24. Intl
   25. JSON
   26. Map
   27. Math
   28. NaN
   29. Number
   30. Object
   31. Promise
   32. Proxy
   33. RangeError
   34. ReferenceError
   35. Reflect
   36. RegExp
   37. Set
   38. SharedArrayBuffer
   39. String
   40. Symbol
   41. SyntaxError
   42. TypeError
   43. TypedArray
   44. URIError
   45. Uint16Array
   46. Uint32Array
   47. Uint8Array
   48. Uint8ClampedArray
   49. WeakMap
   50. WeakSet
   51. WebAssembly
   52. decodeURI()
   53. decodeURIComponent()
   54. encodeURI()
   55. encodeURIComponent()
   56. escape()
   57. eval()
   58. globalThis
   59. isFinite()
   60. isNaN()
   61. null
   62. parseFloat()
   63. parseInt()
   64. undefined
   65. unescape()
   66. uneval()
9. 表达式 & 运算符

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

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