Javascript Escape (\x) + Variable,

Question

Been playing with Javascript and discover the escaped(\\x) which seems interesting.

To clarify I just discover I can convert two digit number into a letter using escape + x,

like "\\x51" output is Q

Been searching about this specific topic but not really sure where to start as I don't even know what they call it, like I know backslash purpose in PHP and Javascript but I don't know what it does when it combines with x

Now my question is, is it possible to add a dynamic number value after the X?

Like I'm trying to create a function like this but It doesn't seem possible, as it looks like it requires to have two characters (number or letter) after x .

function __num_string( num ) {
    return "\x"+num;
}

expectation __num_string( 51 ) would return Q

reality Uncaught SyntaxError: Unexpected token ILLEGAL

I would appreciate any clarification if its possible or not,

Thanks

Answer 1

Yes, it's possible, but there's a better way to do it :

return String.fromCharCode(num);

... where num is a Unicode value of a character you want to get.

Still, technically it's possible to implement your function the way you originally intended, using eval :

function __num_string( num ) {
   if (/^[a-f0-9]{1,2}$/i.test(num)) { 
     return eval('"\\x' + num + '"');
   }
}

Note the difference: num should be a string representing a hexadecimal value.

---

Now about the error you got: this...

'\x'

... is technically invalid string. When parsing string literal, the parser expects the sequence \\x to be followed by [0-9a-f] character range. Yet in your case there's nothing similar there, hence Unexpected token ILLEGAL .

Answer 2

String escaping is a syntactic feature, part of string literals.
It doesn't make sense to do that at runtime.

You're looking for String.fromCharCode , which does exactly what it sounds like.

Answer 3

tldr

• Pass 51₁₆ to String.fromCharCode not 51₁₀
• 51₁₆ is equal to 81₁₀
• Any of the following can be used for conversion
• (81).toString(16)
• 81..toString(16)
• 81.0.toString(16)
• numberVar.toString(16)

After reading the answers and comments in this question an investigation ensued.

Bearing in mind a hypothesis I'd formed from the question

Why do the main solutions, namely String.fromCharCode , seem not to answer the asker's question when tried.

The hypothesis was that the issue lay within the type of argument passed to String.fromCharCode .

The following demonstrates in a node repl the reason why '\\x51' === 'Q' returns true yet String.fromCharCode(51) === 3 also returns true .

> 0x51 // es6 hexadecimal literal
81

> 0x51.toString(16) // Same as (81).toString(16)
'51'

> String.fromCharCode(0x51) // Same as String.fromCharCode(81)
'Q'

> (81).toString(16) // Same as 0x51.toString(16)
51

> String.fromCharCode(81) // Same as String.fromCharCode(0x51)
'Q'

> 0x33 // es6 hexadecimal literal
51

> 0x33.toString(16) // literal to base 16 string
'33'

> String.fromCharCode(0x33)
'3'

> String.fromCharCode(51)
'3'

Just the statements if you wanna try them yourself.

0x51
0x51.toString(16)
String.fromCharCode(0x51)
(81).toString(16)
String.fromCharCode(81)
0x33
0x33.toString(16)
String.fromCharCode(0x33)
String.fromCharCode(51)

It seems as if the method in question takes a hexadecimal string.

If it's a number it's toString() with 16 passed as base is called implicitly.

Below is a function which does the conversion.

function toHex(n) {
  if (typeof n === 'string') return `${parseInt(n, 16)}`;
  return `0x${n}`;
}

It can be used with a number

> String.fromCharCode(toHex(33))
'3'


> String.fromCharCode(toHex(51))
'Q'

It can be used with a string

> String.fromCharCode(toHex('33'))
'3'


> String.fromCharCode(toHex('51'))
'Q'

It can be used with hex digits but only as a string...

> String.fromCharCode(toHex('3f'))
'?'

because hex numeric literals are fine without it.

> String.fromCharCode(0x3f)
'?'