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function stringFunctor(value, fn) {  
    var chars = value.split("")  
    return chars.map(function(char) {  
        return String.fromCharCode(fn(char.charCodeAt(0)))  
    }).join("")
}
function plus1(value) {  
  return value + 1  
}
function minus1(value) {  
  return value - 1  
}
[3,4].map(plus1) // = [4, 5]
stringFunctor("ABC", plus1) // returns "BCD"
stringFunctor(“XYZ”, minus1) // returns “RXY”