How To Round Off In Javascript (Up, Down, To Decimal Places)

Welcome to a short tutorial on how to round off numbers in Javascript. So you have a project that needs to round off a number?

That covers the basics, but let us walk through more examples – Read on!

ⓘ I have included a zip file with all the example source code at the start of this tutorial, so you don’t have to copy-paste everything… Or if you just want to dive straight in.

 

 

QUICK SLIDES

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TABLE OF CONTENTS

 

 

QUICK NOTES

If you spot a bug, feel free to comment below. I try to answer short questions too, but it is one person versus the entire world… If you need answers urgently, please check out my list of websites to get help with programming.

 

EXAMPLE CODE DOWNLOAD

Click here to download the source code, I have released it under the MIT license, so feel free to build on top of it or use it in your own project.

 

 

 

1) ROUND, CEILING, FLOOR, TRUNCATE

// (A) NUMBERS
var numA = 11.4;
var numB = 12.5;

// (B) ROUND OFF
// < 0.5 - ROUND DOWN
// >= 0.5 - ROUND UP
console.log(Math.round(numA)); // 11
console.log(Math.round(numB)); // 13

// (C) CEILING - ALWAYS ROUND UP
console.log(Math.ceil(numA)); // 12
console.log(Math.ceil(numB)); // 13

// (D) FLOOR - ALWAYS ROUND DOWN
console.log(Math.floor(numA)); // 11
console.log(Math.floor(numB)); // 12

// (E) TRUNCATE - NO ROUND OFF, JUST DROP DECIMAL PLACES
console.log(Math.trunc(numA)); // 11
console.log(Math.trunc(numB)); // 12

As in the introduction, there are 4 native Javascript Math functions to round off numbers:

1. Math.round() rounds off to the nearest whole number. That is, decimals with less than 0.5 will be rounded down, rounded up if more than or equals to 0.5.
2. Math.ceil() will always round up to the nearest whole number.
3. Math.floor() will always round down to the nearest whole number.
4. Math.trunc() does not do any rounding, simply drops the decimal places.

 

 

2) ROUNDING TO DECIMAL PLACES

// (A) A BETTER ROUND OFF
//   NUM : NUMBER TO BE ROUNDED
//   PLACES : NUMBER OF DECIMAL PLACES
//   MODE : NONE - ROUND, 0 - FLOOR, 1 - CEILING
function rounder (num, places, mode) {
  // (A1) MULTIPLIER
  var mult = parseInt("1" + "0".repeat(places));
  num = num * mult;
 
  // (A2) ROUND OFF
  if (mode === 1) { num = Math.ceil(num); }
  else if (mode === 0) { num = Math.floor(num); }
  else { num = Math.round(num); }
 
  // (A3) RETURN RESULTS
  return num / mult;
}
 
// (B) TEST
var num = 1.2374;
// 2 DECIMALS ROUND - 1.24
console.log(rounder(num, 2));
// 3 DECIMALS FLOOR - 1.237
console.log(rounder(num, 3, 0));
// 3 DECIMALS CEILING - 1.238
console.log(rounder(num, 3, 1));

So as you can see, all the native round-off functions will only round to whole numbers. To do a “round off to 2 decimal places”, we need to multiply the number by 100, round off, then divide by 100… That is a little inconvenient, so the smarter way is to create an “improved roundoff” function.

• (A1) Multiply the given number by an offset first. I.E. The offset will be 10 if 1 decimal point, 100 if 2 decimal points, 1000 if 3 decimal points, and so on.
• (A2) Use the round() ceil() floor() functions as usual.
• (A3) Finally, “revert” the number by dividing it with the offset.

 

 

 

SUMMARY

Function	Description	Link
Math.round(NUMBER)	Rounds to the nearest whole number.	Click Here
Math.ceil(NUMBER)	Round up to the next whole number.	Click Here
Math.floor(NUMBER)	Round down to the last whole number.	Click Here
Math.trunc(NUMBER)	Remove decimal places.	Click Here

 

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