Rounding and truncating is a bread and butter action for every single developer. It was covered during your first few math lessons in primary school. Hopefully you still remember how it works in the world of numbers.
5 or more? Raise the Score. 4 or less? Let it Rest.
Math. As the name can suggests, it has a collection of properties and methods for mathematical operations on numbers. There is one small difference between
Math and other built-in global objects.
Math isn’t a constructor which means that all properties and methods that belong to it are static (meaning that they need to be called by using Math as an object).
Rounding vs Truncating #
The difference between these two methods is minor but very important to understand. Both of them are methods of approximating a number by dropping decimal places. Rounding approximates a number using a nearby number at a given degree of accuracy. It can occur in two directions: up and down. Rounding up approximates a number towards positive infinity. Rounding down towards negative infinity. Truncating approximates without rounding. In other words, it “rounds” towards zero.
Rounding 3.14159 ≈ 3.1416 Truncating 3.14159 ≈ 3.1415
Math.round()- rounds to the nearest integer (if the fraction is 0.5 or greater - rounds up)
Math.floor()- rounds down
Math.ceil()- rounds up
Math.round(3.14159) // 3 Math.round(3.5) // 4 Math.floor(3.8) // 3 Math.ceil(3.2) // 4
Rounding numbers with decimal precision requires a little bit of calculation and
Math.round(). Optionally we can use the
toFixed() method that belongs to the
Number prototype. The output type of
toFixed() is a
string which needs to be passed to a top-level function called
parseFloat() to return a
number. Unfortunately this seems to be really slow.
Math.round(3.14159 * 100) / 100 // 3.14 3.14159.toFixed(2); // 3.14 returns a string parseFloat(3.14159.toFixed(2)); // 3.14 returns a number
Math.trunc() simply removes all the fractional digits. It takes one argument which is a number. If the argument is a positive number it behaves exactly the same as
Math.floor(). For negative numbers it does the same job as
Math.trunc(3.14159); // 3 Math.trunc(-3.14159); // -3
It’s worth mentioning that the browser support for
Math.trunc() isn’t great. It is part of new ES2015 (yeah, I prefer ES6 too) specification. Have a look at the browser support list:
- Google Chrome >= 38
- Firefox >= 25
- Internet Explorer >= Nope :(
- Opera >= 25
- Safari >= 7.1
Luckily there is a way to use this without ES6 support (thanks to Johny who suggested this solution in comments below). We can use bitwise operators to accomplish this task. Unfortunately there are some restriction as well. All bitwise operations work on signed 32-bit integers. Using them converts a float to an integer. In practice it means that we can safely work up to
2^31−1 (2 147 483 647) which is much less than
Number.MAX_VALUE (1.7976931348623157e+308). This isn’t a great idea for monetary calculations either.
3.14159 | 0; // 3 -3.14159 | 0; // -3 3000000000.1 | 0 // -1294967296 Ups :(
TLTR (too long to read) #
I know, I know - time is money. Lets sum it up.
Math.round()- rounds to the nearest integer
Math.floor()- rounds down towards negative infinity
Math.ceil()- rounds up towards positive infinity
Math.trunc()- rounds up or down towards zero (bad browsers support)