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SyntaxError: applying the 'delete' operator to an unqualified name is deprecated - JavaScript | MDN
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SyntaxError: applying the 'delete' operator to an unqualified name is deprecated

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

In this article

Message

SyntaxError: Delete of an unqualified identifier in strict mode. (V8-based) SyntaxError: applying the 'delete' operator to an unqualified name is deprecated (Firefox) SyntaxError: Cannot delete unqualified property 'a' in strict mode. (Safari)

Error type

SyntaxError in strict mode 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.

The 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.

Examples

Freeing the contents of a variable

Attempting to delete a plain variable throws an error in strict mode:

js
"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:

js
"use strict"; var x; // … x = null; // x can be garbage collected

See also

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  1. JavaScript
  2. Tutorials and guides
  3. JavaScript Guide
    1. Introduction
    2. Grammar and types
    3. Control flow and error handling
    4. Loops and iteration
    5. Functions
    6. Expressions and operators
    7. Numbers and strings
    8. Representing dates & times
    9. Regular expressions
    10. Indexed collections
    11. Keyed collections
    12. Working with objects
    13. Using classes
    14. Using promises
    15. JavaScript typed arrays
    16. Iterators and generators
    17. Resource management
    18. Internationalization
    19. JavaScript modules
  4. Intermediate
    1. Language overview
    2. JavaScript data structures
    3. Equality comparisons and sameness
    4. Enumerability and ownership of properties
    5. Closures
  5. Advanced
    1. Inheritance and the prototype chain
    2. Meta programming
    3. Memory Management
  6. References
  7. Built-in objects
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncDisposableStack
    5. AsyncFunction
    6. AsyncGenerator
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    14. DataView
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    16. decodeURI()
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    18. DisposableStack
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  8. Expressions & operators
    1. Addition (+)
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    4. async function expression
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    7. Bitwise AND (&)
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    40. Logical AND (&&)
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    42. Logical NOT (!)
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    44. Logical OR assignment (||=)
    45. Multiplication (*)
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    49. null
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    56. Remainder (%)
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    70. Unsigned right shift (>>>)
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  11. Classes
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  12. Regular expressions
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  13. Errors
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    3. InternalError: too much recursion
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    5. RangeError: BigInt division by zero
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    8. RangeError: invalid array length
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    14. RangeError: x can't be converted to BigInt because it isn't an integer
    15. ReferenceError: "x" is not defined
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    18. ReferenceError: must call super constructor before using 'this' in derived class constructor
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    20. SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code
    21. SyntaxError: "0"-prefixed octal literals are deprecated
    22. SyntaxError: "use strict" not allowed in function with non-simple parameters
    23. SyntaxError: "x" is a reserved identifier
    24. SyntaxError: \ at end of pattern
    25. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    26. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
    27. SyntaxError: arguments is not valid in fields
    28. SyntaxError: await is only valid in async functions, async generators and modules
    29. SyntaxError: await/yield expression can't be used in parameter
    30. SyntaxError: cannot use ?? unparenthesized within || and && expressions
    31. SyntaxError: character class escape cannot be used in class range in regular expression
    32. SyntaxError: continue must be inside loop
    33. SyntaxError: duplicate capture group name in regular expression
    34. SyntaxError: duplicate formal argument x
    35. SyntaxError: for-in loop head declarations may not have initializers
    36. SyntaxError: function statement requires a name
    37. SyntaxError: functions cannot be labelled
    38. SyntaxError: getter and setter for private name #x should either be both static or non-static
    39. SyntaxError: getter functions must have no arguments
    40. SyntaxError: identifier starts immediately after numeric literal
    41. SyntaxError: illegal character
    42. SyntaxError: import declarations may only appear at top level of a module
    43. SyntaxError: incomplete quantifier in regular expression
    44. SyntaxError: invalid assignment left-hand side
    45. SyntaxError: invalid BigInt syntax
    46. SyntaxError: invalid capture group name in regular expression
    47. SyntaxError: invalid character in class in regular expression
    48. SyntaxError: invalid class set operation in regular expression
    49. SyntaxError: invalid decimal escape in regular expression
    50. SyntaxError: invalid identity escape in regular expression
    51. SyntaxError: invalid named capture reference in regular expression
    52. SyntaxError: invalid property name in regular expression
    53. SyntaxError: invalid range in character class
    54. SyntaxError: invalid regexp group
    55. SyntaxError: invalid regular expression flag "x"
    56. SyntaxError: invalid unicode escape in regular expression
    57. SyntaxError: JSON.parse: bad parsing
    58. SyntaxError: label not found
    59. SyntaxError: missing : after property id
    60. SyntaxError: missing ) after argument list
    61. SyntaxError: missing ) after condition
    62. SyntaxError: missing ] after element list
    63. SyntaxError: missing } after function body
    64. SyntaxError: missing } after property list
    65. SyntaxError: missing = in const declaration
    66. SyntaxError: missing formal parameter
    67. SyntaxError: missing name after . operator
    68. SyntaxError: missing variable name
    69. SyntaxError: negated character class with strings in regular expression
    70. SyntaxError: new keyword cannot be used with an optional chain
    71. SyntaxError: nothing to repeat
    72. SyntaxError: numbers out of order in {} quantifier.
    73. SyntaxError: octal escape sequences can't be used in untagged template literals or in strict mode code
    74. SyntaxError: parameter after rest parameter
    75. SyntaxError: private fields can't be deleted
    76. SyntaxError: property name __proto__ appears more than once in object literal
    77. SyntaxError: raw bracket is not allowed in regular expression with unicode flag
    78. SyntaxError: redeclaration of formal parameter "x"
    79. SyntaxError: reference to undeclared private field or method #x
    80. SyntaxError: rest parameter may not have a default
    81. SyntaxError: return not in function
    82. SyntaxError: setter functions must have one argument
    83. SyntaxError: string literal contains an unescaped line break
    84. SyntaxError: super() is only valid in derived class constructors
    85. SyntaxError: tagged template cannot be used with optional chain
    86. SyntaxError: Unexpected '#' used outside of class body
    87. SyntaxError: Unexpected token
    88. SyntaxError: unlabeled break must be inside loop or switch
    89. SyntaxError: unparenthesized unary expression can't appear on the left-hand side of '**'
    90. SyntaxError: use of super property/member accesses only valid within methods or eval code within methods
    91. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    92. TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed
    93. TypeError: 'x' is not iterable
    94. TypeError: "x" is (not) "y"
    95. TypeError: "x" is not a constructor
    96. TypeError: "x" is not a function
    97. TypeError: "x" is not a non-null object
    98. TypeError: "x" is read-only
    99. TypeError: already executing generator
    100. TypeError: BigInt value can't be serialized in JSON
    101. TypeError: calling a builtin X constructor without new is forbidden
    102. TypeError: can't access/set private field or method: object is not the right class
    103. TypeError: can't assign to property "x" on "y": not an object
    104. TypeError: can't convert BigInt to number
    105. TypeError: can't convert x to BigInt
    106. TypeError: can't define property "x": "obj" is not extensible
    107. TypeError: can't delete non-configurable array element
    108. TypeError: can't redefine non-configurable property "x"
    109. TypeError: can't set prototype of this object
    110. TypeError: can't set prototype: it would cause a prototype chain cycle
    111. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    112. TypeError: class constructors must be invoked with 'new'
    113. TypeError: cyclic object value
    114. TypeError: derived class constructor returned invalid value x
    115. TypeError: getting private setter-only property
    116. TypeError: Initializing an object twice is an error with private fields/methods
    117. TypeError: invalid 'instanceof' operand 'x'
    118. TypeError: invalid Array.prototype.sort argument
    119. TypeError: invalid assignment to const "x"
    120. TypeError: Iterator/AsyncIterator constructor can't be used directly
    121. TypeError: matchAll/replaceAll must be called with a global RegExp
    122. TypeError: More arguments needed
    123. TypeError: null/undefined has no properties
    124. TypeError: property "x" is non-configurable and can't be deleted
    125. TypeError: Reduce of empty array with no initial value
    126. TypeError: setting getter-only property "x"
    127. TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol
    128. TypeError: X.prototype.y called on incompatible type
    129. URIError: malformed URI sequence
    130. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    131. Warning: unreachable code after return statement
  14. Misc
    1. JavaScript technologies overview
    2. Execution model
    3. Lexical grammar
    4. Iteration protocols
    5. Strict mode
    6. Template literals
    7. Trailing commas
    8. Deprecated features

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