Introduction

All programming languages appreciate the significance of numbers in computing and likewise provide features to easily work with them. JavaScript is no way behind - the language comes packed with a ton of functions to facilitate in working with numbers.

In this chapter, we'll explore some methods natively available to JavaScript numbers using the Number() object. Specifically we'll go over the different ways to round numbers and then consider conversions to different numerical bases including binary, octal and hexadecimal.

Rounding numbers

Numbers can be rounded in different ways - they can be rounded to given significant figures, decimal places, or simply converted to standard form. In this section we'll discover three number methods to aid in numerical rounding.

toPrecision()

Starting with the first in the list, the method toPrecision() serves to round a number to given significant figures.

Just to give you an example, the number 0.358 when rounded to the nearest 2 significant figures converts to 0.36. Similarly 157, when converted to 1 sf. gives 200. Basic math!

The number of significant figures, you desire, goes into the method as an argument which in turn throws out the rounded number as a string value.

Consider the following example:

var num = 0.358;
var num2 = num.toPrecison(2); // "0.36"

Below, we experiment with some other arguments to toPrecision(), as applied on different numbers:

var num = 8956
num.toPrecision(3); // "8960"

num = 15.85
num.toPrecision(3); // "15.9"

num = 0.123
num.toPrecision(1); // "0.1"
It is invalid to call toPrecision(0), since every number has to have at least 1 sf.

The fact that toPrecision() returns a string can sometimes trick developers while they are working with it. They might forget to convert back the result to a number, and use it as it is in calculations, ultimately leading to bugs!

Therefore whenever you are working with it, remember to convert its output to a number using the ways discussed in the previous chapter.

Unfortunately this is the case with the next two methods as well.

toFixed()

The method toFixed() rounds a number to a given length of decimal places. In the spec of JavaScript, this is known as the fraction digits of the number.

This method ensures that the result is always fixed at a given decimal point length; hence the name toFixed.

As a quick example, consider the number 3.754 - it has three digits after the decimal point and likewise has a decimal place length of 3. When rounded to two decimal places, it will become 3.75 (since 4 is less than 5).

The method toFixed() is made to work on these digits i.e digits appearing after the decimal point.

Following is an illustration:

var num = 3.754;
var roundedNum = num.toFixed(2); // "3.75"

The same number 3.754 is rounded to 2 decimal places, to give the stringified number "3.75".

Below, we experiment with some other arguments to the method, as applied on num:

num.toFixed(0); // "4"
num.toFixed(1); // "3.8"
num.toFixed(3); // "3.754"
num.toFixed(4); // "3.7540"

First we round the number 3.754 to 0 decimal places i.e to the nearest unit, yielding 4. Second we round the number to 1 decimal place yielding 3.8. Next we round it to 3 decimal places, returning the number 3.754, as it is. Lastly we round it to 4 decimal places to give the number 3.7540.

Trivially, numbers without decimal points will also be rounded to the given decimal precision as you can see below.

var num = 10;
var num2 = num.toFixed(2); // "10.00"

The number 10 is converted to "10.00", with exactly 2 decimal digits.

What will toFixed(3) return for the numbers 1.795, 0.3832, 79.0005 and -10.5?

  • 1.80, 0.383, 79.0 and -10.5
  • "1.80", "0.383", "79.0" and "-10.5"
  • 1.795, 0.383, 79.001 and -10.500
  • "1.795", "0.383", "79.001" and "-10.500"

toExponential()

The method toExponential() rounds a number to the specified length of decimal places together with an exponent value. It is usually used on large numbers to convert them to a short, manageable form.

var num = 1056.7;
num.toExponential(2); // returns "1.06e+3"
num = 945682;
num.toExponential(1); // returns "9.5e+5"
num = 1987;
num.toExponential(0); // returns "2e+3"

Recall from maths: this method can be used to give numbers in standard form.

Converting numbers to different bases

One fairly useful method of numbers in JavaScript is the toString() method.

Although it merely converts a number into a string when called with no arguments, if it's provided with an argument, it will perform conversion of the number into the given base n representation.

The argument has to be a number and is generally called the radix of the converted number. Radix is simply the base, we wish to convert a number into.

Some common bases are base 2, known as binary; base 8, known as octal; and base 16, known as hexadecimal. The number system we use is built on the base 10, and commonly referred to as the decimal number system.

To illustrate this discussion, following we convert the decimal number 15 into all the three representations - binary, octal and hexadecimal:

var num = 15;

// conversion to binary
var binNum = num.toString(2); // "1111"

// conversion to octal
var octNum = num.toString(8); // "17"

// conversion to hexadecimal
var hexNum = num.toString(16); // "f"
In hexadecimal, f stands for the number 15.
Since we can't have the bases 0 or 1 to represent numbers, calling toString(0) or toString(1) will throw errors.

These three conversions are by far the most common out there, but you can still experiment with other bases. See what each one returns!

And once again, note that this method also converts the number into a string; which is, in effect, its sole purpose. Therefore, before working with the converted number just make sure to parse it using parseInt() and the respective base of the number.

Convert the binary number 10110 into hexadecimal representation, and save the result in a variable hexNum.

The following code has been set up for you to start with:

var binNum = "10110";
var hexNum = // put the result here
Note that the final result should be a string.

First we'll need to parse binNum into a number and then convert this to a hexadecimal number.

To parse binNum we'll use parseInt(binNum, 2) and then to convert its output into hexadecimal, we'll use toString(16).

Combining both these functions, we have the following code:

var binNum = "10110";
var hexNum = parseInt(binNum, 2).toString(16); // "16"