// targetNum 需要处理的数字
// targetLonger 需要保留的位数
const formatNum = function (targetNum, targetLonger ) {
targetNum= targetNum.toString()
let index = targetNum.indexOf('.')
if (index != -1) {
targetNum= targetNum.substring(0, targetLonger + index + 1)
} else {
targetNum= targetNum.substring(0)
}
return parseFloat(targetNum).toFixed(targetLonger)
}