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  1. 개발자를 위한 웹 기술
  2. JavaScript
  3. JavaScript 참고서
  4. JavaScript error reference
  5. ReferenceError: assignment to undeclared variable "x"
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ReferenceError: assignment to undeclared variable "x"

메시지

ReferenceError: assignment to undeclared variable "x" (Firefox) ReferenceError: "x" is not defined (Chrome) ReferenceError: Variable undefined in strict mode (Edge)

In this article

에러 형식

엄격 모드(strict mode)에서만 발생하는 ReferenceError 경고.

무엇이 잘못되었을까?

선언되지 않은 변수로 값은 할당되었습니다. var 키워드가 없이 할당이 된 것입니다. 선언된 변수와 선언되지 않은 변수 사이에는 차이가 있는데, 이는 예상치 못한 결과를 가져오며, 때문에 JavaScript 엄격모드에서는 에러를 발생시키고 있습니다.

선언된 변수와 선언되지 않은 변수에 대하여 기억해야 할 세 가지:

  • 선언된 변수는 선언된 실행 맥락 내에서 요구됩니다. 선언되지 않은 변수는 항상 전역의 특성을 띱니다.
  • 선언된 변수는 코드가 실행되기 전에 생성됩니다. 선언되지 않은 변수는 실행을 위해 할당이 일어날 때까지 존재하지 않습니다.
  • 선언된 변수는 실행 맥락 내(함수나 전역적인)에서 변경 불가한 요소입니다. 선언되지 않은 변수는 변경이 가능합니다. (삭제 될 수도 있습니다.)

더 많은 설명과 예제를 필요로 한다면 이 var 참조문서 페이지를 보세요.

선언되지 않은 변수 할당에 대한 에러는 엄격 모드(strict mode code)에서만 발생합니다. 비-엄격 코드에서는 조용히 묵인됩니다.

예

허용되지 않는 경우

이런 경우에는, 변수 "bar"는 선언되지 않은 변수가 됩니다.

js
function foo() { "use strict"; bar = true; } foo(); // ReferenceError: assignment to undeclared variable bar

허용되는 경우

"bar" 를 선언된 변수로 만들기 위해서, var 키워드를 변수명 앞에 붙여줍니다.

js
function foo() { "use strict"; var bar = true; } foo();

참조

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  1. JavaScript
  2. 자습서:
  3. JavaScript 안내서
    1. 안내서 소개
    2. 문법과 자료형
    3. 제어 흐름과 오류 처리
    4. 루프와 반복
    5. 함수
    6. 표현식과 연산자
    7. Numbers and strings
    8. Representing dates & times
    9. 정규 표현식
    10. index 기반의 컬렉션
    11. key 기반의 컬렉션
    12. 객체로 작업하기
    13. class 사용하기
    14. promise 사용하기
    15. JavaScript 형식화 배열
    16. 반복기와 생성기
    17. Resource management
    18. Internationalization
    19. JavaScript 모듈
  4. 중급서
    1. JavaScript 재입문하기 (JS ​자습서)
    2. JavaScript의 타입과 자료구조
    3. 동치 비교 및 동일성
    4. 열거성과 속성의 소유권
    5. 클로저
  5. 고급서
    1. 상속과 프로토타입
    2. 메타 프로그래밍
    3. JavaScript 메모리 관리
  6. 참고서:
  7. 표준 내장 객체
    1. AggregateError
    2. Array
    3. ArrayBuffer
    4. AsyncDisposableStack
    5. AsyncFunction
    6. AsyncGenerator
    7. AsyncGeneratorFunction
    8. AsyncIterator
    9. Atomics
    10. BigInt
    11. BigInt64Array
    12. BigUint64Array
    13. Boolean
    14. DataView
    15. Date
    16. decodeURI()
    17. decodeURIComponent()
    18. DisposableStack
    19. encodeURI()
    20. encodeURIComponent()
    21. Error
    22. escape()
    23. eval()
    24. EvalError
    25. FinalizationRegistry
    26. Float16Array
    27. Float32Array
    28. Float64Array
    29. Function
    30. Generator
    31. GeneratorFunction
    32. globalThis
    33. Infinity
    34. Int8Array
    35. Int16Array
    36. Int32Array
    37. InternalError
    38. Intl
    39. isFinite()
    40. isNaN()
    41. Iterator
    42. JSON
    43. Map
    44. Math
    45. NaN
    46. Number
    47. Object
    48. parseFloat()
    49. parseInt()
    50. Promise
    51. Proxy
    52. RangeError
    53. ReferenceError
    54. Reflect
    55. RegExp
    56. Set
    57. SharedArrayBuffer
    58. String
    59. SuppressedError
    60. Symbol
    61. SyntaxError
    62. Temporal
    63. TypedArray
    64. TypeError
    65. Uint8Array
    66. Uint8ClampedArray
    67. Uint16Array
    68. Uint32Array
    69. undefined
    70. unescape()
    71. URIError
    72. WeakMap
    73. WeakRef
    74. WeakSet
  8. 표현식 및 연산자
    1. 더하기 (+)
    2. 더하기 할당 (+=)
    3. 할당 연산자 (=)
    4. async function 표현식
    5. async function* 표현식
    6. await
    7. AND 비트연산(&)
    8. 비트 AND 할당 연산 (&=)
    9. 비트 NOT (~)
    10. OR 비트연산(|)
    11. Bitwise OR assignment (|=)
    12. Bitwise XOR (^)
    13. Bitwise XOR assignment (^=)
    14. class 식
    15. 쉼표 연산자
    16. 조건 (삼항) 연산자
    17. 감소 연산자 (--)
    18. delete 연산자
    19. 구조 분해 할당
    20. 나누기 (/)
    21. 나누기 할당 (/=)
    22. 동등 연산자(==)
    23. 거듭제곱 (**)
    24. 거듭제곱 할당 (**=)
    25. 함수 표현식
    26. function* expression
    27. Greater than (>)
    28. Greater than or equal (>=)
    29. 그룹 연산자
    30. import.meta
      1. import.meta.resolve()
    31. import()
    32. in 연산자
    33. 증가 연산자(++)
    34. 부등 연산자(!=)
    35. instanceof
    36. 왼쪽 시프트 (<<)
    37. 왼쪽 시프트 할당 연산자 (<<=)
    38. Less than (<)
    39. Less than or equal (<=)
    40. 논리적 AND (&&)
    41. Logical AND assignment (&&=)
    42. 논리적 NOT (!)
    43. Logical OR (||)
    44. Logical OR assignment (||=)
    45. 곱하기 (*)
    46. 곱하기 할당 (*=)
    47. new operator
    48. new.target
    49. null
    50. Nullish coalescing assignment (??=)
    51. Nullish coalescing operator
    52. 객체 초기자
    53. 연산자 우선순위
    54. Optional chaining
    55. 속성 접근자
    56. 나머지 (%)
    57. 나머지 할당 (%=)
    58. 오른쪽 시프트 (>>)
    59. Right shift assignment (>>=)
    60. 전개 구문
    61. 일치 연산자 (===)
    62. 불일치 연산자 (!==)
    63. 빼기 (-)
    64. 빼기 할당 (-=)
    65. super
    66. this
    67. typeof
    68. 단항 부정 (-)
    69. 단항 더하기 (+)
    70. Unsigned right shift (>>>)
    71. Unsigned 오른쪽 시프트 할당 연산자 (>>>=)
    72. void
    73. yield
    74. yield*
  9. 문과 선언문
    1. async function
    2. async function*
    3. await using
    4. block
    5. break
    6. class
    7. const
    8. continue
    9. debugger
    10. do...while
    11. empty
    12. export
    13. Expression statement
    14. for
    15. for await...of
    16. for...in
    17. for...of
    18. 함수 선언
    19. function*
    20. if...else
    21. import
      1. Import attributes
    22. label
    23. let
    24. return
    25. switch
    26. throw
    27. try...catch
    28. using
    29. var
    30. while
    31. with
  10. 함수
    1. 화살표 함수 표현식
    2. 기본값 매개변수
    3. 접근자
    4. 메서드 정의
    5. 나머지 매개변수
    6. 설정자
    7. arguments 객체
      1. arguments[@@iterator]()
      2. arguments.callee
      3. arguments.length
  11. Classes
    1. constructor
    2. extends
    3. Private class fields
    4. Public class fields
    5. static
    6. Class static initialization blocks
  12. 정규 표현식
    1. Backreference: \1, \2
    2. Capturing group: (...)
    3. Character class escape: \d, \D, \w, \W, \s, \S
    4. Character class: [...], [^...]
    5. Character escape: \n, \u{...}
    6. Disjunction: |
    7. Input boundary assertion: ^, $
    8. Literal character: a, b
    9. Lookahead assertion: (?=...), (?!...)
    10. Lookbehind assertion: (?<=...), (?<!...)
    11. Modifier: (?ims-ims:...)
    12. Named backreference: \k<name>
    13. Named capturing group: (?<name>...)
    14. Non-capturing group: (?:...)
    15. Quantifier: *, +, ?, {n}, {n,}, {n,m}
    16. Unicode character class escape: \p{...}, \P{...}
    17. Wildcard: .
    18. Word boundary assertion: \b, \B
  13. 오류
    1. AggregateError: No Promise in Promise.any was resolved
    2. Error: Permission denied to access property "x"
    3. InternalError: too much recursion
    4. RangeError: argument is not a valid code point
    5. RangeError: BigInt division by zero
    6. RangeError: BigInt negative exponent
    7. RangeError: form must be one of 'NFC', 'NFD', 'NFKC', or 'NFKD'
    8. RangeError: invalid array length
    9. RangeError: invalid date
    10. RangeError: precision is out of range
    11. RangeError: radix must be an integer
    12. RangeError: repeat count must be less than infinity
    13. RangeError: repeat count must be non-negative
    14. RangeError: x can't be converted to BigInt because it isn't an integer
    15. ReferenceError: "x" is not defined
    16. ReferenceError: assignment to undeclared variable "x"
    17. ReferenceError: can't access lexical declaration`X' before initialization
    18. ReferenceError: must call super constructor before using 'this' in derived class constructor
    19. ReferenceError: super() called twice in derived class constructor
    20. SyntaxError: 'arguments'/'eval' can't be defined or assigned to in strict mode code
    21. SyntaxError: "0"-prefixed octal literals and octal escape seq. 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. ReferenceError: 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: Malformed 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. ReferenceError: deprecated caller or arguments usage
    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: "x" 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. 기타
    1. JavaScript 소개
    2. Execution model
    3. Lexical 문법
    4. Iteration 프로토콜
    5. 엄격한 모드
    6. 템플릿 리터럴
    7. Trailing commas
    8. 제외 예정 기능과 제외된 기능

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