Convert from Instagram Shortcode to Id and Back using big-integer

InstagramIdConvert.js

var bigint = require( 'big-integer' )

var lower = 'abcdefghijklmnopqrstuvwxyz';
var upper = lower.toUpperCase();
var numbers = '0123456789'
var ig_alphabet =  upper + lower + numbers + '-_'
var bigint_alphabet = numbers + lower

function toShortcode( longid )
{
	var o = bigint( longid ).toString( 64 )
	return o.replace(/<(\d+)>|(\w)/g, (m,m1,m2) =>
	{
		return ig_alphabet.charAt( ( m1 )
		  ? parseInt( m1 ) 
		  : bigint_alphabet.indexOf( m2 ) )
	});
}

function fromShortcode( shortcode )
{
	var o = shortcode.replace( /\S/g, m =>
	{
		var c = ig_alphabet.indexOf( m )
		var b = bigint_alphabet.charAt( c ) 
		return ( b != "" ) ? b : `<${c}>`
	} )	
	return bigint( o, 64 ).toString( 10 )
}

toShortcode(  '908540701891980503' ) // s.b. 'ybyPRoQWzX'
fromShortcode( 'ybyPRoQWzX' ) // s.b. '908540701891980503'

Here is a simpler way to do the decode part using built-in BigInt and Base64URL handling. This is typescript but you can easily remove the types if you want.

function instagramShortCodeToId(shortCode: string) {
  const paddedShortCode = shortCode.padStart(12, 'A');
  //const bytes = Buffer.from(paddedShortCode, 'base64url'); // if you want to use Node
  const bytes = Uint8Array.fromBase64(paddedShortCode, { alphabet: 'base64url' });

  let id = 0n; // this is a BigInt
  for (let i = 0; i < bytes.length; i++) {
    // shift by one byte and add next byte
    id = (id << 8n) + BigInt(bytes[i]);
  }

  return id;
}

and a bonus of extracting the timestamp in milliseconds

function instagramIdToTimestampMs(id: bigint) {
  const instagramEpochMs = 1314220021721n;

  const offsetTimestamp = id >> (64n - 41n);
  const timestampMs = offsetTimestamp + instagramEpochMs;

  return Number(timestampMs);
}

function instagramShortCodeToTimestampMs(shortCode: string) {
  return instagramIdToTimestampMs(instagramShortCodeToId(shortCode));
}

implementing the encode should be pretty straightforward using the same approach.