Javascript arithmetic operator not working

Because subTotal is a string (since you used toFixed), make it

var sub_total = parseFloat((rate * quantity) + sub_total);
sub_total = sub_total.toFixed(2);

Demo

var sub_total = 0;
for (var i = 0; i < 3; i++) {
  var rate = parseFloat(10).toFixed(2);
  var quantity = parseFloat(15).toFixed(2);
  var sub_total = parseFloat((rate * quantity) + sub_total);
}
sub_total = sub_total.toFixed(2);
console.log(sub_total);

Edit

Expected Output:

450.00

Change the for-loop back to

for (var i = 0; i <= 3; i++) {

Could this be the answer to your question?

As the others already said: toFixed returns a String

var sub_total = 0;
  for(var i = 0; i <= 3; i++){
  var rate = parseFloat(10); 
  var quantity = parseFloat(15); 
  var sub_total = parseFloat((rate * quantity) + sub_total);
}
console.log(sub_total);

Hope below code is helpful.

 var sub_total = 0;
 for(var i = 0; i <= 3; i++){
  var rate = parseFloat(10).toFixed(2); 
  var quantity = parseFloat(15).toFixed(2); 
  sub_total+ =parseFloat(rate) * parseFloat(quantity) + 
  parseFloat(sub_total);

  }
 console.log(sub_total);

You could use numbers without parseFloat. At the end, you could format the number.

You need only to iterate three times and assign the unchanging values only once.

var sub_total = 0,
    rate = 10,
    quantity = 15,
    i;

for (i = 0; i < 3; i++){
    sub_total += rate * quantity;
}

console.log(sub_total.toFixed(2));

For Expected Output: 450.00

You have to loop only <3 then you will be get required output.

See blow Demo

var sub_total = 0,
  rate = 10,
  quantity = 15,
  i = 0;

for (; i < 3; i++) {
  sub_total += (rate * quantity);
}


console.log(sub_total.toFixed(2));

I have removed "<=" and replaced it with "<" since previous case leads to another iteration,because of that you get 600.00 .Also I have used toFixed method at the end, since initially we need to calculate the value of sub_total first then we have to convert it to a string having two decimals.

 var sub_total = 0;

  for(var i = 0; i < 3; i++){
    var rate = parseFloat(10).toFixed(2); 
    var quantity = parseFloat(15).toFixed(2); 
    sub_total = parseFloat((rate * quantity) + sub_total);
  }

    console.log(sub_total.toFixed(2));