unicode字符串求utf8字节长度：https://segmentfault.com/a/1190000005794963

1.js

let sizeof = function(str) {
    let total = 0, charCode, i, len
    for (i = 0, len = str.length; i < len; i++) {
        charCode = str.charCodeAt(i)
        if (charCode <= 0x007f) {
            total += 1
        } else if (charCode <= 0x07ff) {
            total += 2
        } else if (charCode <= 0xffff) {
            total += 3
        } else if (charCode <= 0x1fffff) {
            total += 4
        } else if (charCode <= 0x3ffffff) {
            total += 5
        } else {
            total += 6
        }
    }
    return total
}

2.js

let get_send_string = function(str) {
    let total = 0, charCode, i, len, back = []
    for (i = 0, len = str.length; i < len; i++) {
        charCode = str.charCodeAt(i)
        if (charCode <= 0x007f) {
            total += 1
            back.push(charCode)
        } else if (charCode <= 0x07ff) {
            total += 2
            back.push((192 | (31 & (charCode >> 6))))
            back.push((128 | (63 & charCode)))
        } else {
            total += 3
            back.push((224 | (15 & (charCode >> 12))));
            back.push((128 | (63 & (charCode >> 6))));
            back.push((128 | (63 & charCode)))
        }
    }
    for (i = 0; i < back.length; i++) {
        back[i] &= 0xff
    }
    if (total <= 0xff) {
        return new Uint8Array([0, total].concat(back))
    } else {
        return new Uint8Array([total >> 8, total & 0xff].concat(back))
    }
}
get_send_string(chunk).buffer instanceof ArrayBuffer