emoji.js

'use strict'

const blocks = {
  symbols: '[\u2000-\u2bff]',
  cjk: '[\u2e80-\u9fff]',
  surrogates: '[\ud800-\udbff]',
  selectors: '[\ufe00-\ufe0f]'
};

const valid = new RegExp(`^${Object.keys(blocks).map(key => blocks[key]).join('|')}$`);

function emoji(char) {
  const buffer = new Buffer(char);

  let length = 0;

  if (buffer[0] >> 5 === 6)
    length = 2;

  if (buffer[0] >> 4 === 14)
    length = 3;

  if (buffer[0] >> 3 === 30)
    length = 4;

  if (!length)
    return false;

  for (let index = 1; index < length; index++)
    if (buffer[index] >> 6 !== 2)
      if (!valid.test(buffer[index].toString()))
        return false;

  return true;
}

console.log(emoji('😀')); // true
console.log(emoji('x')); // false
console.log(emoji('🇦🇩')); // true