js保留n位小数，不四舍五入

     function isNumber(val) {

            let result = /^[-+]?(([0-9]+)([.]([0-9]+))?|([.]([0-9]+))?)$/.test(val);

        　　 return result;

        }

        function numberFixed(val,n=2) { 

            if(isNumber(val)){

                let str = val + '';

                if(str.includes('.')){

                    let arr = str.split('.');

                    let result = arr[0] + '.' + arr[1].substring(0,n);

                    return Number(result);

                } else{

                    return Number(val);

                }

                return Number(str.substring());

            }else{

                return val;

            }

        }

        numberFixed(0.2,6);//0.2

        numberFixed('0.643'); //0.64

        numberFixed(240.3536363625,6);//240.353636

        numberFixed('hello'); // hello