How do you map-replace characters in Javascript similar to the 'tr' function in Perl?

There isn't a built-in equivalent, but you can get close to one with replace:

data = data.replace(/[\-_]/g, function (m) {
    return {
        '-': '+',
        '_': '/'
    }[m];
});

I can't vouch for 'efficient' but this uses a regex and a callback to provide the replacement character.

function tr( text, search, replace ) {
    // Make the search string a regex.
    var regex = RegExp( '[' + search + ']', 'g' );
    var t = text.replace( regex, 
            function( chr ) {
                // Get the position of the found character in the search string.
                var ind = search.indexOf( chr );
                // Get the corresponding character from the replace string.
                var r = replace.charAt( ind );
                return r;
            } );
    return t;
}

For long strings of search and replacement characters, it might be worth putting them in a hash and have the function return from that. ie, tr/abcd/QRST/ becomes the hash { a: Q, b: R, c: S, d: T } and the callback returns hash[ chr ].

Method:

String.prototype.mapReplace = function(map) {
    var regex = [];
    for(var key in map)
        regex.push(key.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&"));
    return this.replace(new RegExp(regex.join('|'),"g"),function(word){
        return map[word];
    });
};

A perfect example:

var s = "I think Peak rocks!"
s.mapReplace({"I think":"Actually","rocks":"sucks"})
// console: "Actually Peak sucks!"