javascript - Math.flooring a scientific notation at a certain decimal -

i trying math.floor scientific notation, @ 1 point number gets big , current method doesn't work anymore. using atm

var nr = (number+"").length - 4; if( nr > 1 ) {     nr = math.pow( 10, nr );     number= math.floor(number/nr)*nr;     number= number.toexponential(3); } 

when becomes scientific notation default, think that's e20+, .length method doesn't work anymore since length returns isn't accurate. can think of work around, , that's find out number after e, , update nr math.floor properly, seems work simple. here's example number 8.420960987929105e+79 want turn 8.420e+79, there way can math.floor third decimal point always, no matter number is? stands when use toexponential(3) rounds number. numbers can high e+200 easily, need easier way of doing i'm doing.

edit: managed find work around works besides connor peet's answer wants options

var nr = 8.420960987929105e+79+""; var nr1 = nr.substr(0,4); var nr2 = nr.substr(4, nr.length); var finalnr = number(nr1 + 0 + nr2).toexponential(3); 

this way more of hack, adds 0 after 4th number when toexponential rounds up, gets 'floored' pretty much.

i wrote little snippet round number number of significant figures time ago. might find useful

function sigfigs(num, figures) {     var delta = math.pow(10, math.ceil(math.log(num) / math.log(10)) - figures);      return math.round(num / delta) * delta; }  sigfigs(number, 3); // => 8.420e+79