Encode / decode UTF8 array in JavaScript (original: https://gist.github.com/joni/3760795)

fromUTF8Array.js

function fromUTF8Array(data) { // array of bytes
    let str = '',
        i;

    for (i = 0; i < data.length; i++) {
        let value = data[i];

        if (value < 0x80) {
            str += String.fromCharCode(value);
        } else if (value > 0xBF && value < 0xE0) {
            str += String.fromCharCode((value & 0x1F) << 6 | data[i + 1] & 0x3F);
            i += 1;
        } else if (value > 0xDF && value < 0xF0) {
            str += String.fromCharCode((value & 0x0F) << 12 | (data[i + 1] & 0x3F) << 6 | data[i + 2] & 0x3F);
            i += 2;
        } else {
            // surrogate pair
            let charCode = ((value & 0x07) << 18 | (data[i + 1] & 0x3F) << 12 | (data[i + 2] & 0x3F) << 6 | data[i + 3] & 0x3F) - 0x010000;

            str += String.fromCharCode(charCode >> 10 | 0xD800, charCode & 0x03FF | 0xDC00); 
            i += 3;
        }
    }

    return str;
}

toUTF8Array.js

function toUTF8Array(str) {
    let utf8 = [];
    
    for (let i=0; i < str.length; i++) {
        let charcode = str.charCodeAt(i);
        
        if (charcode < 0x80) {
            utf8.push(charcode);
        } else if (charcode < 0x800) {
            utf8.push(0xc0 | (charcode >> 6), 
                      0x80 | (charcode & 0x3f));
        } else if (charcode < 0xd800 || charcode >= 0xe000) {
            utf8.push(0xe0 | (charcode >> 12), 
                      0x80 | ((charcode>>6) & 0x3f), 
                      0x80 | (charcode & 0x3f));
       
        // surrogate pair
        } else {
            i++;
            // UTF-16 encodes 0x10000-0x10FFFF by
            // subtracting 0x10000 and splitting the
            // 20 bits of 0x0-0xFFFFF into two halves
            charcode = (((charcode & 0x3ff) << 10)
                         | (str.charCodeAt(i) & 0x3ff))
                         + 0x010000;
            utf8.push(0xf0 | (charcode >>18), 
                      0x80 | ((charcode>>12) & 0x3f), 
                      0x80 | ((charcode>>6) & 0x3f), 
                      0x80 | (charcode & 0x3f));
        }
    }
    
    return utf8;
}