js保留小数（不四舍五入）

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function formatDecimal(num, decimal) {           num = num.toString()           let index = num.indexOf('.')           if (index !== -1) {               num = num.substring(0, decimal + index + 1)           } else {               num = num.substring(0)           }           return parseFloat(num).toFixed(decimal)       }

　　

console.log(formatDecimal(3.1415926,6))      --->   3.141592