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  5. SyntaxError: missing = in const declaration

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The JavaScript exception "missing = in const declaration" occurs when a const declaration was not given a value in the same statement (like const RED_FLAG; ). You need to provide one ( const RED_FLAG = '#ff0' ).

Message 

SyntaxError: Const must be initalized (Edge)
SyntaxError: missing = in const declaration (Firefox)
SyntaxError: Missing initializer in const declaration (Chrome)

Error type

SyntaxError

What went wrong?

A constant is a value that cannot be altered by the program during normal execution. It cannot change through re-assignment, and it can't be redeclared. In JavaScript, constants are declared using the const keyword. An initializer for a constant is required; that is, you must specify its value in the same statement in which it's declared (which makes sense, given that it can't be changed later).

范例

Missing const initializer

不像 var or let , you must specify a value for a const declaration. This throws: 

const COLUMNS;
// SyntaxError: missing = in const declaration

Fixing the error

There are multiple options to fix this error. Check what was intended to be achieved with the constant in question.

Adding a constant value

Specify the constant value in the same statement in which it's declared: 

const COLUMNS = 80;

const , let or var ?

Do not use const if you weren't meaning to declare a constant. Maybe you meant to declare a block-scoped variable with let or global variable with var . Both don't require an initial value. 

let columns;

另请参阅

元数据

  • 最后修改：
  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
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  10. 语句 & 声明
    1. Legacy generator function
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  11. 函数
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    5. 自变量对象
    6. getter
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  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