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SyntaxError: "0"-prefixed octal literals are deprecated

The JavaScript strict mode-only exception "0-prefixed octal literals are deprecated; use the "0o" prefix instead" occurs when deprecated octal literals (0 followed by digits) are used.

In this article

Message

SyntaxError: Octal literals are not allowed in strict mode. (V8-based) SyntaxError: Decimals with leading zeros are not allowed in strict mode. (V8-based) SyntaxError: Unexpected number (V8-based) SyntaxError: "0"-prefixed octal literals are deprecated; use the "0o" prefix instead (Firefox) SyntaxError: Decimal integer literals with a leading zero are forbidden in strict mode (Safari)

Error type

SyntaxError in strict mode only.

What went wrong?

Octal literals are deprecated. When you prefix a decimal integer with 0, you actually change it to an octal literal, which may be surprising. The standardized syntax uses a leading zero followed by a lowercase or uppercase Latin letter "O" (0o or 0O).

Leading zeros are always forbidden, even when the literal is not valid octal literal syntax (such as when the literal contains the digits 8 or 9, or it has a decimal point). A number literal may only start with 0 if that 0 is its units place.

Examples

"0"-prefixed octal literals

js
"use strict"; 03; // SyntaxError: "0"-prefixed octal literals are deprecated; use the "0o" prefix instead

Valid octal numbers

Use a leading zero followed by the letter "o" or "O":

js
0o3;

See also

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  1. JavaScript
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    44. SyntaxError: invalid assignment left-hand side
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    46. SyntaxError: invalid capture group name in regular expression
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    49. SyntaxError: invalid decimal escape in regular expression
    50. SyntaxError: invalid identity escape in regular expression
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    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"
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    86. SyntaxError: Unexpected '#' used outside of class body
    87. SyntaxError: Unexpected token
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    91. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    92. TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed
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    99. TypeError: already executing generator
    100. TypeError: BigInt value can't be serialized in JSON
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    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
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    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
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    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
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    125. TypeError: Reduce of empty array with no initial value
    126. TypeError: setting getter-only property "x"
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  14. Misc
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    2. Execution model
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    4. Iteration protocols
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    6. Template literals
    7. Trailing commas
    8. Deprecated features

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