SOURCE

//计算字符串长度包含中文
 function getByteLen(val) {
    var len = 0;
    for (var i = 0; i < val.length; i++) {
            var a = val.charAt(i);
            if (a.match(/[^\x00-\xff]/ig) != null) 
        {
            len += 2;
        }
        else
        {
            len += 1;
        }
    }
    return len;
}

function a(val,num){
    let arr = []
    for (var i = 0; i < val.length; i++) {
            var a = val.charAt(i);
            
            if (a.match(/[^\x00-\xff]/ig) != null) 
        {
            arr.push(a,'')
        }
        else
        {
            arr.push(a)
        }
    }
    return arr.filter((v,i)=>i<num).join('');
}
console.log(a('123啊阿斯顿发3',7))