isq

A javascript implementation of the International System of Quantities, ISO-80000.

The latest documentation can be read at Read the Docs. The release history and change notes are on github.

Features

• Uncertainty - isq('123.456(4) km')
• SI notation - isq('1.234 56(4) × 10² km')
• ASCII notation - isq('1.234 56(4) x 10^2 km')
• Conversion - isq('25 m/s').to('km/h')
• Symbolic expressions - isq('W/(m² sr)')
• Pluggable - isq.config.Number = require('big.js')

Getting started

Install with npm

> npm install isq

Usage

Include the package

var isq = require('isq')

Create a quantity with the function returned by the isq module

var a = isq('1.03 kg'),
    b = isq('1.03(1) kg') // 1.03±0.01 kg
    kg = isq('kg'); // 1 kg
    

A Quantity is converted to a string with the toString() method. It uses a heuristic to determine the best symbol to use.

isq('5 * 10^-6 s').toString()  // 5 µs
isq('50 000 V/A').toString()   // 50 kΩ

Javascript does not allow overiding of operators, so named methods are used. The methods are also chainable. For example, the hypotenuse of a triangle is

var a = isq('1.2 m'),
    b = isq('80 cm'), // 0.8 m
    c = a.pow(2).plus(b.pow(2)).sqrt(); // ~ 1.44 m

Numbers

Create a number with isq.Number

var a = isq.Number(0.3),
    b = isq.Number('0.1'),
    c = isq.Number('0.03(1)'), // 0.03±0.01
    d = isq.Number(0.03, 0.01); // same as c

It is most likely easier to just create a unitless Quantity, as in:

var c = isq('0.03(1)');     // 0.03±0.01

Rounding errors

Rounding and precision errors are notorious in Javascript. For example 0.3 - 0.1 produces 0.19999999999999998 and NOT 0.2. ISQ can be configured to use a 'big number' package that avoids these issues.

isq.config.Number = require('big.js');
isq.Number(0.3).minus(isq.Number(0.1)) // 0.2

Uncertainty

Uncertainity, or margin of error, describes the imperfect nature of a measurement. Typically, it is the standard deviation of actual measurements. Anytime a calculation is performed, propagation of uncertainity is also performed to determine the uncertainty of the result.

let a = isq('1.2(2) m'),   // 1.2±0.2 m
    b = isq('1.3(3) m'),   // 1.3±0.3 m
    length = a.plus(b);    // 2.5±0.4 m

When comparing uncertain numbers, the uncertainity of both values is taken into consideration. Equality Is the difference of the two values within the resulting uncertainty?

Command line

A command line interface (isq) is also available. Type isq --help for the most current usage.

  Usage: isq [options] expression

  SI quantities

  Options:

    -h, --help          output usage information
    -V, --version       output the version number
    -b, --base          output in SI base units
    --to <unit>         convert to another unit
    --number <package>  the number [package] to use. Defaults to "big.js". 
                        "js" will use standard javascript numbers.

To convert a quantity try something like:

isq 25 m/s --to km/h
 

License

The MIT license.

Copyright © 2015-2016 Richard Schneider ([email protected])