*   * handle long strings, intercept and add ellipses   * note: the length of the half Angle is 1, and the length of the whole Angle is 2   *   * pStr: string   * pLen: intercept length   *   * return: the truncated string   *

function autoAddEllipsis(pStr, pLen) {

    var _ret = cutString(pStr, pLen);
    var _cutFlag = _ret.cutflag;
    var _cutStringn = _ret.cutstring;

    if ("1" == _cutFlag) {
        return _cutStringn + "...";
    } else {
        return _cutStringn;
    }
}

*   * gets a string of the specified length   * note: the length of the half Angle is 1, and the length of the whole Angle is 2   *   * pStr: string   * pLen: intercept length   *   * return: the truncated string   *

function cutString(pStr, pLen) {

    //Original string length
    var _strLen = pStr.length;

    var _tmpCode;

    var _cutString;

    //By default, the returned string is part of the original string
    var _cutFlag = "1";

    var _lenCount = 0;

    var _ret = false;

    if (_strLen <= pLen/2) {
        _cutString = pStr;
        _ret = true;
    }

    if (!_ret) {
        for (var i = 0; i < _strLen ; i++ ) {
            if (isFull(pStr.charAt(i))) {
                _lenCount += 2;
            } else {
                _lenCount += 1;
            }

            if (_lenCount > pLen) {
                _cutString = pStr.substring(0, i);
                _ret = true;
                break;
            } else if (_lenCount == pLen) {
                _cutString = pStr.substring(0, i + 1);
                _ret = true;
                break;
            }
        }
    }

    if (!_ret) {
        _cutString = pStr;
        _ret = true;
    }

    if (_cutString.length == _strLen) {
        _cutFlag = "0";
    }

    return {"cutstring":_cutString, "cutflag":_cutFlag};
}

*   * to determine whether it is a full Angle   *   * pChar: a string of length 1   * return: tbtrue: full Angle   *                   False: half Angle   *

function isFull (pChar) {
  for (var i = 0; i < pChar.strLen ; i++ ) {    
    if ((pChar.charCodeAt(i) > 128)) {
        return true;
    } else {
        return false;
    }
}
}

Use cases:

testStr = " test 1 string ";
autoAddEllipsis(testStr, 1); //"Test..."
autoAddEllipsis(testStr, 2); //"Test..."
autoAddEllipsis(testStr, 3); //"Test..."
autoAddEllipsis(testStr, 4); //"Test..."
autoAddEllipsis(testStr, 5); //"The test 1..."
autoAddEllipsis(testStr, 6); //"The test 1..."
autoAddEllipsis(testStr, 7); //"Test 1 word..."