javaScript toFixed()四舍五入失真

```
Number.prototype.toFixed = function (n) {
    if (n != undefined && (isNaN(n) || Number(n) > 17 || Number(n) < 0)) {
        throw new Error("输入正确的精度范围")
    }
    // 拆分小数点整数和小数
    var num = this
    var f = ""
    if (Number(num) < 0) {
        num = -Number(num)
        f = "-"
    }
    var numList = num.toString().split(".")
    // 整数
    var iN = numList[0]
    // 小数
    var dN = numList[1]
    n = parseInt(n)
    if (isNaN(n) || Number(n) === 0) {
        // 0或者不填的时候，按0来处理
        if (dN === undefined) {
            return f + num + ""
        }
        var idN = Number(dN.toString().substr(0, 1))
        if (idN >= 5) {
            if (Number(iN) < 0) {
                iN = Number(iN) - 1
            } else {
                iN = Number(iN) + 1
            }
        }
        return f + iN + ""
    } else {
        var dNL = dN === undefined ? 0 : dN.length
        if (dNL < n) {
            // 如果小数位不够的话，那就补全
            var oldN = num.toString().indexOf(".") > -1 ? num : num + "."
            var a = Number(n) - dNL
            while (a > 0) {
                oldN += "0"
                a--
            }
            return f + oldN
        }
        // 正常
        var olddN = dN.toString().substring(0, n)
        var dN1 = Number(dN.toString().substring(0, n))
        var dN2 = Number(dN.toString().substring(n, n + 1))
        if (dN2 >= 5) {
            dN1 += 1
            if (olddN.length - dN1.toString().length > 0) {
                var olddNL = olddN.length - dN1.toString().length
                while (olddNL > 0) {
                    dN1 = "0" + dN1
                    olddNL--
                }
            }
            if (dN1.toString().length > n) {
                if (Number(iN) < 0) {
                    iN = Number(iN) - 1
                } else {
                    iN = Number(iN) + 1
                }
                dN1 = "0"
                while (n > 1) {
                    dN1 += "0"
                    n--
                }
            }
        } else {
            dN1 = olddN
        }
        return f + iN + "." + dN1
    }
}
```

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