# SI Prefixes in JavaScript Most people are familiar with a few of the more common prefixes used before many units to denote a fraction or a multiple of the unit - kilograms, megabytes, centimetres etc.. As well as these there a number of less well known ones, going right up to yotta and right down to yocto.

A while ago I wrote a post on SI Prefixes in Python to list all the prefixes along with their corresponding powers and multipliers. When I started writing an equivalent in JavaScript I soon hit a brick wall when I realised that JavaScript's Number type couldn't provide the precision needed.

Searching around for a solution to the problem I discovered the excellent math.js library which, amongst many other things, provides us with a BigNumber type and I thoroughly recommend reading the documentation here. This post covers my simple little JavaScript project to list the numeric SI prefixes.

This project consists of a JavaScript file called siprefixes.js as well as an HTML page and a few ancilliary files. You can download them as a zip or clone/download from Github if you prefer. The zip and repo include the minimized version of math.js called (you guessed it!) math.min.js.

Let's look at the source code.

siprefixes.js

{
SIprefixes();
}

function SIprefixes()
{
const ten = math.bignumber(10);

const base10 = [{"prefix": "yotta", "power": 24},
{"prefix": "zetta", "power": 21},
{"prefix": "exa", "power": 18},
{"prefix": "peta", "power": 15},
{"prefix": "tera", "power": 12},
{"prefix": "giga", "power": 9},
{"prefix": "mega", "power": 6},
{"prefix": "kilo", "power": 3},
{"prefix": "hecto", "power": 2},
{"prefix": "deca", "power": 1},
{"prefix": "(none)", "power": 0},
{"prefix": "deci", "power": -1},
{"prefix": "centi", "power": -2},
{"prefix": "milli", "power": -3},
{"prefix": "micro", "power": -6},
{"prefix": "nano", "power": -9},
{"prefix": "pico", "power": -12},
{"prefix": "femto", "power": -15},
{"prefix": "atto", "power": -18},
{"prefix": "zepto", "power": -21},
{"prefix": "yocto", "power": -24}];

for(let p of base10)
{
writeToConsole("10^" + p.power.toString().padEnd(4, " ") + "=");
writeToConsole(math.format(math.pow(ten, p.power), {notation: 'fixed'}).padStart(27, " ") + "<br />");
}
}

In window.onload we simply call SIprefixes() which firstly initializes a constant to a math.js BigNumber with value 10 which of course is the base of the decimal system.

Next we create an array of objects containing the SI prefixes and their corresponding powers. (If you have heard of more than half of them give yourself a pat on the back.)

Finally we iterate the array. The first line in the loop simply prints out the prefix name, padded to format the output neatly. The second line prints out the power or exponent, again padded to keep things lined up neatly.

The last line is where it all happens. It is somewhat convoluted so I'll break it down.

math.pow(ten, p.power)

This is straightforward, it mirrors JavaScript's own Math.pow but can handle BigNumber types.

math.format(math.pow(ten, p.power), {notation: 'fixed'})

Here we wrap the call to math.pow in a call to math.format which formats the number as a string in accordance with second argument, in this case 'fixed'. Without this very large or very small numbers would be displayed in scientific notation, for example yotta or 10^24 would be shown as 1e+24 which is perfectly correct of course but not what I wanted for this project.

math.format(math.pow(ten, p.power), {notation: 'fixed'}).padStart(27, " ") 