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Asked: 2026-06-12T14:12:37+00:00

I find that when I escape numbers that start from 0 to 7 and

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I find that when I escape numbers that start from 0 to 7 and alert them, I get a weird symbol:

alert( "\0" ); // or \1, \2, \3...\7

Weird character

This is only visible when I alert it, but document.write and console.log won’t show it. Escaping numbers greater than 7 will appear fine. I’m using the latest version of Chrome. Why am I getting this weird character? Thanks.

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1 Answer

  1. Editorial Team

    How string literals are parsed in general is described in section 7.8.4.

    However, the behaviour you see is described in Annex B.1.2. This section is about octal escape sequences in older ES versions, which still seems to be supported. The resulting character is defined as follows:

    • The CV of OctalEscapeSequence :: OctalDigit [lookahead ∉ DecimalDigit] is the character whose code unit value is the MV of the OctalDigit.
    • The CV of OctalEscapeSequence :: ZeroToThree OctalDigit [lookahead ∉ DecimalDigit] is the character whose code unit value is (8 times the MV of the ZeroToThree) plus the MV of the OctalDigit.
    • The CV of OctalEscapeSequence :: FourToSeven OctalDigit is the character whose code unit value is (8 times the MV of the FourToSeven) plus the MV of the OctalDigit.
    • The CV of OctalEscapeSequence :: ZeroToThree OctalDigit OctalDigit is the character whose code unit value is (64 (that is, 82) times the MV of the ZeroToThree) plus (8 times the MV of the first OctalDigit) plus the MV of the second OctalDigit.

    \x (\xx, \xxx) is only interpreted as octal sequence if x is an octal digit, i.e. between 0 and 7.

    So, all the characters \0 – \7 are actually control characters. Higher values refer to other characters, for example \101 is A.

    I cannot tell you why alert is showing a strange character and the console does not show anything… that’s probably an implementation detail.

    Fun fact: Octal escape sequences are not allowed in strict mode.

    Fun fact #2: \0 is actually not an octal escape sequence and will still work in strict mode, since it has its own production rule (see section 7.8.4). OTHA, \00 is an octal sequence and will throw an error in strict mode.

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