Javascript: Find out previous letter in alphabet

If I got a Letter in JavaScript, I'd like to find out the previous letter in alphabetic order, so if input i开发者_如何学JAVAs "C", output must be "B". Are there any standard solutions or do i have to create some special functions?

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var ch = 'b';
String.fromCharCode(ch.charCodeAt(0) - 1); // 'a'

And if you wanted to loop around the alphabet just do a check specifically for 'a' -- loop to 'z' if it is, otherwise use the method above.

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This should work in some cases, you might need to tweak it a bit:

function prevLetter(letter) {
    return String.fromCharCode(letter.charCodeAt(0) - 1);
}

If letter is A, the result is @, so you need to add some sanity checking if you want it to be foolproof. Otherwise should do the job just fine.

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The full function from Tatu's comment would be

function prevLetter(letter) {
    if (letter === 'a'){ return 'z'; }
    if (letter === 'A'){ return 'Z'; }
    return String.fromCharCode(letter.charCodeAt(0) - 1);
}

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Something like this should work.

function prevLetter(letter) {
    var code = letter.charCodeAt(0);
    var baseLetter = "A".charCodeAt(0); 
    if (code>"Z".charCodeAt(0)) {
        var baseLetter = "a".charCodeAt(0); 
    }
    return String.fromCharCode((code-baseLetter+25)%26+baseLetter);
}