gitter-badger / isq

International System of Quantities, ISO-80000

isq

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

The latest documentation can be read at Read the Docs.

Features

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

Getting started

Install with npm

> npm install isq

Usage

Include the package

var SI = require('isq')

Create a number with SI.Number

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

Create a quantity with SI.Quantity with an optional number and a unit

var a = SI.Quantity('1.03 kg'),
    b = SI.Quantity('1.03(1) kg') // 1.03±0.01 kg
    kg = SI.Quantity('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.

SI.Quantity('5 * 10^-6 s').toString()  // 5 µs
SI.Quantity('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 = SI.Quantity('1.2 m'),
    b = SI.Quantity('80 cm'), // 0.8 m
    c = a.pow(2).plus(b.pow(2)).sqrt(); // ~ 1.44 m

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.

SI.config.Number = require('big.js');
SI.Number(0.3).minus(SI.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 = SI.Number('1.2(2)'),   // 1.2±0.2
    b = SI.Number('1.3(3)'),   // 1.3±0.3
    length = a.plus(b);        // 2.5±0.4

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?

License

The MIT license.

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