How to increase my program speed?

How to increase my program speed?
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#1

I wrote a javascript program to return largest prime cofactor.My input is 600851475143.
But when i use my code in browser console than it takes very very long time.I want to increase my program speed and i tried many thing but it still it takes very large time.What should i do?
Here is my code:-

//object to store inforamtion realated to this program
//I have predeclared property instead of declaring them in loop to increase my program speed
var dataObject = {
	a: [],
	rem: 0,
	x:0
};

//writing function to find cofacators of a number and store those in an array
function checkFactor(n){
	var rem=0;
	var a=[];
	for(var i=1;i<=n;i++){
		rem = n%i;
		if(rem===0) a.push(i);
	}
	return a;
}

function operate(n){
	var x; // this  variable will be finally assigned largest prime cofactors
	var subArray;
	var mainArray = checkFactor(n); //array containing factors of n
	if(mainArray.length===2) return mainArray[1];
	for(var i=0;i<mainArray.length;i++){
		 subArray = checkFactor(mainArray[i]);
		if(subArray.length===2) x=mainArray[i];
}
	return x;
}

#2

I cleaned up your code.
You need to use triple backticks to post code to the forum.
See this post for details.