How many bytes in a JavaScript string?

If you're using node.js, there is a simpler solution using buffers :

function getBinarySize(string) {
    return Buffer.byteLength(string, 'utf8');
}

There is a npm lib for that : https://www.npmjs.org/package/utf8-binary-cutter (from yours faithfully)

This function will return the byte size of any UTF-8 string you pass to it.

function byteCount(s) {
    return encodeURI(s).split(/%..|./).length - 1;
}

Source

JavaScript engines are free to use UCS-2 or UTF-16 internally. Most engines that I know of use UTF-16, but whatever choice they made, it’s just an implementation detail that won’t affect the language’s characteristics.

The ECMAScript/JavaScript language itself, however, exposes characters according to UCS-2, not UTF-16.

Source

You can use the Blob to get the string size in bytes.

Examples:

console.info(
  new Blob(['😂']).size,                             // 4
  new Blob(['👍']).size,                             // 4
  new Blob(['😂👍']).size,                           // 8
  new Blob(['👍😂']).size,                           // 8
  new Blob(['I\'m a string']).size,                  // 12

  // from Premasagar correction of Lauri's answer for
  // strings containing lone characters in the surrogate pair range:
  // https://stackoverflow.com/a/39488643/6225838
  new Blob([String.fromCharCode(55555)]).size,       // 3
  new Blob([String.fromCharCode(55555, 57000)]).size // 4 (not 6)
);