SOURCE

//整数
const str1 = '8';
console.log(str1.padStart(9, '0')); // "000000008"

const fullNumber = '123456789';
const maskedNumber = fullNumber.slice(-3).padStart(fullNumber.length, '*');
console.log(maskedNumber);// ******789
const maskedNumber2 = fullNumber.slice(0,3).padEnd(fullNumber.length, '*');
console.log(maskedNumber2);// 123******

//小数
console.log((5).toPrecision(4)); //5.000

//toPrecision是四舍六入五成双
console.log(5.555.toPrecision(3)); // "5.55"

//要想四舍五入得手动改造
const demo = (data) =>{
    let _data = Number(data);
    if(isNaN(_data)){
        return false;
    }
    let intLength = Math.trunc(_data).toString().length;
    let fullLength = intLength + 2;//小数点保留2位,算出总位数
    let newData = Math.round(_data * 100) / 100;
    return Number(newData).toPrecision(fullLength)
}

console.log(demo(5.555));//  5.56
console.log(demo(5.5));//   5.50