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  1. Tecnologia Web para desenvolvedores
  2. JavaScript
  3. Referência JavaScript
  4. JavaScript error reference
  5. SyntaxError: redeclaration of formal parameter "x"
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SyntaxError: redeclaration of formal parameter "x"

Mensagem

SyntaxError: Let/Const redeclaration (Edge) SyntaxError: redeclaration of formal parameter "x" (Firefox) SyntaxError: Identifier "x" has already been declared (Chrome)

In this article

Tipo de Erro

SyntaxError

O que deu errado?

O mesmo nome de variável ocorre como parâmetro da função e é então redeclarada atribuindo let no corpo da função. Redeclarando a mesma variável dentro da mesma função ou no escopo do bloco usando let não é permitido em JavaScript.

Exemplos

Neste caso, a variável "arg" redeclara o argumento.

js
function f(arg) { let arg = "foo"; } // SyntaxError: redeclaration of formal parameter "arg"

Se você quiser mudar o valor de "arg" no corpo da função, você pode fazer, mas você não precisa declarar novamente a mesma variável. Em outras palavras: você pode omitir a palavra chave let. Se você quiser criar uma nova variável, você precisa renomea-la, pois isso conflita com o parâmetro da função.

js
function f(arg) { arg = "foo"; } function f(arg) { let bar = "foo"; }

Notas de compatibilidade

Veja também

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  1. JavaScript
  2. Tutoriais
  3. Guia de JavaScript
    1. Introdução
    2. Gramática e tipos
    3. Controle de fluxo e tratamento de erros
    4. Repetição e iteração
    5. Funções
    6. Expressões e operadores
    7. Numbers and strings
    8. Representing dates & times
    9. Expressões Regulares
    10. Coleções indexadas
    11. Coleções com chave (chave-valor)
    12. Trabalhando com objetos
    13. Using classes
    14. Usando "promises"
    15. Arranjos tipados em JavaScript
    16. Iteradores e geradores
    17. Resource management
    18. Internationalization
    19. JavaScript modules
  4. Intermediário
    1. Language overview
    2. Estruturas de dados de JavaScript
    3. Comparadores de igualdade e semelhança
    4. Enumerabilidade e domínio de propriedades
    5. Closures
  5. Avançado
    1. Herança e a cadeia de "prototype"
    2. Metaprogramação
    3. Gerenciamento de memória
  6. Referências
  7. Objetos pré-definidos
    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. Número
    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. Símbolo
    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. Expressões e operadores
    1. Adição (+)
    2. Addition assignment (+=)
    3. Assignment (=)
    4. Expressão da função async
    5. async function* expression
    6. await
    7. Bitwise AND (&)
    8. Bitwise AND assignment (&=)
    9. Bitwise NOT (~)
    10. Bitwise OR (|)
    11. Bitwise OR assignment (|=)
    12. Bitwise XOR (^)
    13. Bitwise XOR assignment (^=)
    14. class expression
    15. Operador Vírgula
    16. Operador Condicional Ternário
    17. Decrement (--)
    18. delete
    19. Atribuição via desestruturação (destructuring assignment)
    20. Divisão (/)
    21. Division assignment (/=)
    22. Equality (==)
    23. Operador Exponencial
    24. Exponentiation assignment (**=)
    25. Expressão de função (function expression)
    26. Expressão function*
    27. Greater than (>)
    28. Greater than or equal (>=)
    29. Operador de Agrupamento
    30. import.meta
      1. import.meta.resolve()
    31. import()
    32. in operator
    33. Increment (++)
    34. Desigualdade (!=)
    35. instanceof
    36. Left shift (<<)
    37. Left shift assignment (<<=)
    38. Less than (<)
    39. Less than or equal (<=)
    40. AND lógico (&&)
    41. Logical AND assignment (&&=)
    42. NOT lógico (!)
    43. Logical OR (||)
    44. Logical OR assignment (||=)
    45. Multiplication (*)
    46. Multiplication assignment (*=)
    47. new operator
    48. new.target
    49. null
    50. Nullish coalescing assignment (??=)
    51. Operador de coalescência nula
    52. Inicializador de Objeto
    53. Precedência de Operadores
    54. Encadeamento opcional
    55. Assessores de propriedade
    56. Remainder (%)
    57. Remainder assignment (%=)
    58. Right shift (>>)
    59. Right shift assignment (>>=)
    60. Sintaxe de Espalhamento
    61. Strict equality (===)
    62. Strict inequality (!==)
    63. Subtração (-)
    64. Subtraction assignment (-=)
    65. super
    66. this
    67. typeof
    68. Unary negation (-)
    69. Unary plus (+)
    70. Unsigned right shift (>>>)
    71. Unsigned right shift assignment (>>>=)
    72. void operator
    73. yield
    74. yield*
  9. Afirmações e declarações
    1. Funções assíncronas
    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. Declaração de função
    19. function*
    20. if...else
    21. import
      1. Import attributes
    22. label (Etiqueta)
    23. let
    24. return
    25. switch
    26. throw
    27. try...catch
    28. using
    29. var
    30. while
    31. with
  10. Funções
    1. Arrow functions
    2. Parâmetros Predefinidos
    3. getter
    4. Definições de Método
    5. Parâmetros Rest
    6. setter
    7. Arguments object
      1. [Symbol.iterator]()
      2. callee
      3. length
  11. Classes
    1. constructor
    2. extends
    3. Private elements
    4. Public class fields
    5. static
    6. Static initialization blocks
  12. Expressões Regulares
    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. Escape de classes de caracteres unicode: \p{...}, \P{...}
    17. Wildcard: .
    18. Word boundary assertion: \b, \B
  13. Erros
    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" não está definido
    16. ReferenceError: Atribuição à variável não declarada "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 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: uma declaração na cabeça de um laço for-of não pode ter um inicializador
    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: as declarações da cabeça do laço for-in podem não ter inicializadores
    36. Erro de sintaxe: declaração de função requer um nome
    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: flag de expressão regular inválida "x"
    56. SyntaxError: invalid unicode escape in regular expression
    57. SyntaxError: JSON.parse: bad parsing
    58. SyntaxError: label not found
    59. Erro de Sintaxe: faltando : depois da propriedade id
    60. Erro de sintaxe falta ) depois da lista de argumentos
    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: unterminated string literal
    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: atribuição inválida para 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. Mais conteúdo
    1. Visão geral das Tecnologias JavaScript
    2. Execution model
    3. Gramática léxica
    4. Protocolos de iteração
    5. Strict mode - Modo estrito
    6. Template literals
    7. Trailing commas
    8. Funcionalidades depreciadas

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