javascript - avoiding bugs in .toFixed()

toFixed.js

function toFixed(num, dec) {
    var result = Math.round(num*Math.pow(10,dec))/Math.pow(10,dec),    
        $arr = (result+[]).split('.'),
        $int = $arr[0] + '.',
        $dec = $arr[1] || '0';
    
    return $int + $dec + (Math.pow(10,(dec - $dec.length))+[]).substr(1);
}

//OR

Number.prototype.toFixed = function(dec){
    var result = Math.round(this*Math.pow(10,dec))/Math.pow(10,dec),    
        $arr = (result+[]).split('.'),
        $int = $arr[0] + '.',
        $dec = $arr[1] || '0';
    
    return $int + $dec + (Math.pow(10,(dec - $dec.length))+[]).substr(1);
}