How to reverse String.fromCharCode?

'H'.charCodeAt(0)

Use charCodeAt:

var str = 'H';
var charcode = str.charCodeAt(0);

String.fromCharCode accepts multiple arguments, so this is valid:

const binaryArray = [10, 24] // ...
str = String.fromCharCode(...binaryArray)

In case you're looking for the opposite of that (like I was), this might come in handy:

const binaryArray = str
  .split('')
  .reduce((acc, next) =>
    [...acc, next.charCodeAt(0)],
    []
  )

If you have to consider encoding, note that the charCodeAt method from String object will default to the utf16 encoding. You can use the string_decoder object and the encoding parameter of the Node.js Buffer object to apply a specific encoding.

The charCodeAt method provides only ascii/utf-16 encoding even with utf-8 encoded String object :

str=new String(new Buffer([195,169]))
// -> [String: 'é']
str.charCodeAt(0)
// -> 233

Building the string with fromCharCode would not be right as it expects UTF-16 :

myStr=String.fromCharCode(50089, 65, 233)
// -> '쎩Aé'
Buffer.from(myStr, 'utf-8')
// -> <Buffer ec 8e a9 41 c3 a9>
Buffer.from(myStr, 'ascii')
// -> <Buffer a9 41 e9>
myStr.charCodeAt(0)
// -> 50089
myStr.charCodeAt(2)
// -> 233

Here is a simple example to encode and decode UTF8 bytes and ASCII bytes :

var dec=new string_decoder.StringDecoder('utf-8');
dec.write(Buffer.from([65,67,195,169,98]));
// -> 'ACéb'
var theBytes=new Buffer('aéé','utf-8'); 
// -> <Buffer 61 c3 a9 c3 a9>

var dec=new string_decoder.StringDecoder('ascii')
dec.write(Buffer.from([65,67,195,169,98]))
// -> 'ACC)b'
var theBytes=new Buffer('aéé','ascii')
// -> <Buffer 61 e9 e9>

The StringDecoder.write method will return you a String object from the provided bytes buffer.

The encoding parameter of the Buffer object gives you a way to get a Buffer object feed with the encoded bytes from a provided String.

So to get the ASCII encoding of char 'H' :

new Buffer('H','ascii');
// -> <Buffer 48>
new Buffer('H','ascii')[0];
// -> 72

That way, you could also handle multi bytes encoded chars like that:

new Buffer('é','ascii');
// -> <Buffer e9>
arr=new Buffer('é','ascii');
// -> <Buffer e9>
code=arr.readUIntBE(0,arr.length);
// -> 233
code.toString(16);
// -> 'e9'

new Buffer('é','utf-8');
// -> <Buffer c3 a9>
arr=new Buffer('é','utf-8');
// -> <Buffer c3 a9>
code=arr.readUIntBE(0,arr.length);
// -> 50089
code.toString(16)
// -> 'c3a9'