In this article I will describe how to round numbers using JavaScript and how to easily remember names of these functions.

JavaScript has a few methods in Math object to transform any number or number-as-string to integer (real number).

Math is a global object that contains methods related to math. We will take a look on four of them: Math.round, Math.ceil, Math.floor and Math.trunc.

All these methods accept a number as an argument and return the nearest integer.

Math.ceil

Math.ceil() — always rounds a number to larger integer value. Method name ceil is an abbreviation of the word ceiling — it is what above you if you in building :)

Association: ceil = above, up;

Math.ceil(-1.3); // Output: -1
Math.ceil(-1.7); // Output: -1
Math.ceil(2.3); // Output: 3
Math.ceil(2.7); // Output: 3

Math.floor

Math.floor() — always rounds a number to nearest smaller integer value. It also English word that could be obvious for native speakers. It is the lower surface of a room, on which one may walk.

Association: floor = down, lower, ground.

Math.floor(-1.3); // Output: -2
Math.floor(-1.7); // Output: -2
Math.floor(2.3); // Output: 2
Math.floor(2.7); // Output: 2

Math.round

Math.round() — rounds number to the closest number depending on fractional part. If fraction bigger or equal .5 it will rounds up. If fraction part is less it will rounds down.

Math.round(-1.3); // Output: -2
Math.round(-1.7); // Output: -1
Math.round(2.3); // Output: 2
Math.round(2.7); // Output: 3

Math.trunc

Math.trunc() — just cuts off fraction part of number or truncates. For positive integers work the same way as floor but for negative behaviour is different

Math.trunc(-1.3); // Output: -1
Math.trunc(-1.7); // Output: -1
Math.trunc(2.3); // Output: 2
Math.trunc(2.7); // Output: 2

All these methods when called with anything, but a number or null as an argument, always return NaN

Interactive playground

This is interactive form you could play with rounding numbers (don’t forget about negative numbers and invalid inputs)

-2
↔
Math.round
-3
←−∞
Math.floor
→∞
Math.ceil
-2
↔
Math.trunc
-2

Conclusion

Rounding numbers and dealing with numbers is very important part of any programming languages on any position and memorising this functions really speed up your growing as a developer.

Hope article will be useful for you.