Answer by chrislau for String length in bytes in JavaScript

This would work for BMP and SIP/SMP characters.

    String.prototype.lengthInUtf8 = function() {        var asciiLength = this.match(/[\u0000-\u007f]/g) ? this.match(/[\u0000-\u007f]/g).length : 0;        var multiByteLength = encodeURI(this.replace(/[\u0000-\u007f]/g)).match(/%/g) ? encodeURI(this.replace(/[\u0000-\u007f]/g, '')).match(/%/g).length : 0;        return asciiLength + multiByteLength;    }'test'.lengthInUtf8();    // returns 4'\u{2f894}'.lengthInUtf8();    // returns 4'سلامعلیکم'.lengthInUtf8();    // returns 19, each Arabic/Persian alphabet character takes 2 bytes. '你好，JavaScript 世界'.lengthInUtf8();    // returns 26, each Chinese character/punctuation takes 3 bytes.