Dynamic vs Direct Octal Character Representation in JavaScript

Question

If I was to create a three digit octal character string the following way:

x = "\\" + a + b + c

Where a is either 0 or 1, and b and c are both a number from 0 to 7, why would alert(x) result in \141 for example, and not a which is alert("\141")?

Second to this, how would you get the original created character code to result in its respective ASCII character?

Answer 1

Because you're escaping the backslash. Also I'm not sure if JavaScript supports octal encoding, I know it supports hex using \u61 (equivalent hex for octal 141) but if you want to specify it dynamically like that, try using something like this:

// a, b, and c are octal digits

var octal = parseInt('' + a + b + c, 8);

var x = String.fromCharCode(octal);

Answer 2

why would alert(x) result in \141 for example, and not a which is alert("\141")?

Because inside a string literal, a \\ and \ are different things. The first creates an actual backslash in the string, whereas in your second example, the backslash introduces an octal escape sequence. It's the same reason \\n in a string literal is a backslash followed by the letter n, but \n is a newline.

Or somewhat more simply: Because you're doing nothing that tries to reinterpret what you've created as a string literal. It's already a string, and nothing tries to evaluate the characters inside it using the string literal rules a second time.

how would you get the original created character code to result in its respective ASCII character

If I'm understanding the question correctly, two ways:

1. Strip off the \\, convert the remainder of the string to a number via parseInt(..., 8) (octal), and then use String.fromCharCode.

2. Use eval. But don't do this, do #1

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Side note: Octal escape sequences aren't part of standard JavaScript, although in loose mode implementations are allowed, but not required, to support them. They're not allowed in strict mode at all.