The only sane way of printing to the console is using console.log(). The drawback is that it inserts a newline everytime you call it, so you can't just print every character separately. You have to build a string and print it afterwards. As you noted, you can convert every byte to char with String.fromCharCode().

Also, functions like Uint8ClampedArray.prototype.join() are not yet implemented.

var a = new Uint8ClampedArray([37, 38, 39]);
console.log([].map.call(a, function(v){
    return String.fromCharCode(v);
}).join());

Note that Uint8ClampedArray is not available in PhantomJS 1.x.

console.log() doesn't work in binary mode, so it will split characters above the 127 code point into two bytes when printed. This happens only when printing. You can check with

String.fromCharCode(147).length; // 1

If you are willing to write a file, you can use the fs module to write it in binary:

var a = new Uint8ClampedArray([37, 38, 39]),
    fs = require('fs');

var b = [].map.call(a, function(v){
    return String.fromCharCode(v);
}).join("");

fs.write("img.dat", b, "wb");