Round Off Numbers In Javascript (Up Down Nearest)

Once upon a time, a student confidently brushed off “round off numbers” in Javascript, because it is “so easy”. Then, she froze in a quiz when asked to “round a number to 3 decimal places” and “round to nearest 5 cents”. Well, let Master Coffee show you how, let’s go.

 

CODE DOWNLOAD

ZIP File

I have released this under the MIT license, feel free to use it in your own project – Personal or commercial. Some form of credits will be nice though. 🙂

 

 

VIDEO TUTORIAL

 

1) ROUND OFF

1-round.js
// (PART A) ROUND OFF
var demoA = Math.round(0.5);
console.log(demoA); // 1

var demoB = Math.round(0.49);
console.log(demoB); // 0

The “default round off” in Javascript that everyone should be aware of – Math.round(). How it works is very simple:

  • Round up if the first decimal place is more than or equal to 0.5.
  • Round down if the first decimal place is less than 0.5.

 

 

2) FLOOR & CEILING

2-ceil-floor.js
// (PART B) FLOOR & CEILING
var demoA = Math.floor(1.98);
console.log(demoA); // 1

var demoB = Math.ceil(2.34);
console.log(demoB); // 3

Floor and ceiling is the next “Javascript default round off” that everyone should know.

  • floor() Drops the decimal value regardless.
  • ceil() Rounds up to the nearest whole number, regardless of decimal value.

 

 

3) ROUND TO NEAREST N DECIMAL PLACE

3-decimal-place.js
// (PART C) ROUND OFF TO N DECIMAL PLACES
// (C1) ROUND OFF FUNCTION
var roundto = (num, places) => {
  var offset = +("1" + ("0".repeat(places)));
  return Math.round(num * offset) / offset;
};

// (C2) ROUND OFF DEMO
var demoA = 1.345;
console.log(roundto(demoA, 2)); // 1.35

var demoB = 2.123456;
console.log(roundto(demoB, 3)); // 2.123

var demoC = 3.3;
console.log(roundto(demoC, 99)); // 3.3

How do we round off to 1, 2, 3, N decimal places? The formula is easy:

  • To round off to 1 decimal places – NUM = Math.round(NUM * 10) / 10
  • To round off to 2 decimal places – NUM = Math.round(NUM * 100) / 100
  • To round off to 3 decimal places – NUM = Math.round(NUM * 1000) / 1000

We can “automate” this by creating a simple function to calculate the “10 offset”:

  • OFFSET = PARSE INTEGER ("1" + (REPEAT "0" ACCORDING TO NUMBER OF DECIMAL PLACES))
  • NUM = Math.round(NUM * OFFSET) / OFFSET

 

 

4) ROUND TO NEAREST 5 CENT

4-nearest-cent.js
// (PART D) ROUND OFF TO NEAREST 5 CENT
// (D1) ROUND OFF FUNCTION
var roundcent = amount => {
  // (D1-1) SPLIT & CHECK AMOUNT
  let split = amount.toString().split(".");
  if (split.length!=2 || split[1].length<2) { return amount; }

  // (D1-2) NUMBER YOGA
  split[0] = +split[0]; // whole number
  split[2] = +split[1][1]; // 2nd decimal
  split[1] = +split[1][0]; // 1st decimal

  // (D1-3) ROUND OFF
  // less than 0.03, round down to 0
  // between 0.03 and 0.07, round to 0.05
  // more than 0.07, round up to 0.1
  if (split[2] < 3) { split[2] = 0; }
  else if (split[2] >=3 && split[2]<=7) {split[2] = 5; }
  else {
    split[1]++; split[2] = 0;
    if (split[1]==10) {
      split[0]++; split[1] = 0;
    }
  }

  // (D1-4) RETURN RESULT
  return +`${split[0]}.${split[1]}${split[2]}`;
};

// (D2) ROUND OFF DEMO
console.log(roundcent(1.92)); // 1.9
console.log(roundcent(1.94)); // 1.95
console.log(roundcent(1.97)); // 1.95
console.log(roundcent(1.98)); // 2

If you need “specialized round off”, the only way is to create your own function. Here is a “round off to nearest 5 cent” that I created for a POS.

  • (D1-1) Split the given amount into:
    • split[0] whole numbers.
    • split[1] decimal numbers.
    • If the given amount is a whole number or only has 1 decimal place – Return it as it is.
  • (D1-2) Further split the numbers.
    • split[0] Whole number.
    • split[1] First decimal place.
    • split[2] Second decimal place.
  • (D1-3) Round off.
    • If DECIMAL < 0.03, round down to 0.
    • If DECIMAL >= 0.03 && DECIMAL <= 0.07, round to 0.05.
    • If DECIMAL >= 0.08, round up to 0.1 – If the first decimal place is over 9, increment the whole number.
  • (D1-4) Return the result, AMOUNT = PARSE INTEGER (split[0] . split[1] split[2])

 

 

THE END – BEWARE OF NUMBER & STRING DATA TYPE

That’s all for this quick tutorial and examples. Just one common beginner pitfall to highlight before end:

  • This is a string – "1" + "1" = "11"
  • This is a number – 1 + 1 = 2
  • If your calculations of “wrong”, remember to parse the variables to numbers.

 

CHEAT SHEET

Round Off Numbers in Javascript (click to enlarge)