js实现toFixed截取效果

Number.prototype.toFixed = function(fractionDigits) {
    var f = parseInt(fractionDigits) || ;
    if( f < - || f >  ) {
        throw new RangeError("Precision of " + f + " fractional digits is out of range");
    }
    var x = Number(this);
    if( isNaN(x) ) {
        return "NaN";
    }
    var s = "";
    if(x <= ) {
        s = "-";
        x = -x;
    }
    if( x >= Math.pow(, ) ) {
        return s + x.toString();
    }
    var m;
// 10. Let n be an integer for which the exact mathematical value of
// n ÷ 10^f - x is as close to zero as possible.
// If there are two such n, pick the larger n.
    n = Math.round(x * Math.pow(, f) );

    if( n ==  ) {
        m = "";
    }
    else {
        // let m be the string consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
        m =  n.toString();
    }
    if( f ==  ) {
        return s + m;
    }
    var k = m.length;
    if(k <= f) {
        var z = Math.pow(, f+-k).toString().substring();
        m = z + m;
        k = f+;
    }
    if(f > ) {
        var a = m.substring(, k-f);
        var b = m.substring(k-f);
        m = a + "." + b;
    }
    return s + m;
};

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