Multi-unit Quantity Views

This is a node library to easily change the displayed unit of a quantity (Length, Area, Speed, Pressure...), and easily set the quantity value in any predefined unit.

Caution: This library should not be used in code requiring high accuracy or efficient/massive unit conversions. It is intended as an auxiliary resource to implement user interfaces, mainly associated to data binding techniques as in angular or react.

Installation

To install the library, run:

$ npm install quantity-views -g

or add it as a dependency to your project.

Basic usage

Install the library and try it in a interactive node session

$ node

First, require one of the builtin quantity classes (more about builtin quantities below)

> Length = require('quantity-views').Length
[Function: Length]

and instanciate it as an object representing for example a width, with a value of 1 (1 what? More below)

> width = new Length(value=1, name='width')
Length { _value: 1, name: 'width', precision: 15, unit: 'metre' }

Check the available units that width can display/be set

> width.units
[ 'metre',
  'centimetre',
  'millimetre',
  'inch',
  'foot',
  'footUS',
  'yard',
  'kilometre',
  'nauticalMille',
  'mille',
  'milleUS' ]

Obtain our width in metres (we discover that the initial values is in metres)

> width.metre
'1.00000000000000'

and then in centimetres

> width.centimetre
'100.000000000000'

Now set our width in centimetres

> width.centimetre = 3.4
3.4

and get it in metres

> width.metre
'0.0340000000000000'

or inches

> width.inch
'1.33858267716535'

Set width as one inch

> width.inch = 1
1

and obtain it in millimetres

> width.millimetre
'25.4000000000000'

Inputs can be strings strictly representing floats

> width.foot = '2.3'
'2.3'
> width.metre
'0.701040000000000'

Weird strings are allowed

> width.yard = '34hhjp'
'34hhjp'

but the quantity then is Not a Number

> width.metre
'NaN'

Show symbols for every unit

> Array.from(width.units, v => [v, width.getSymbol(v)])
[ [ 'metre', 'm' ],
  [ 'centimetre', 'cm' ],
  [ 'millimetre', 'mm' ],
  [ 'inch', 'in' ],
  [ 'foot', 'ft' ],
  [ 'footUS', 'ftUS' ],
  [ 'yard', 'yd' ],
  [ 'kilometre', 'km' ],
  [ 'nauticalMille', 'NM' ],
  [ 'mille', 'mi' ],
  [ 'milleUS', 'miUS' ] ]

Builtin quantities

The quantity classes available on require(quantity-views), are

• Length
• Area
• Volume
• Speed
• Acceleration
• Mass
• Density
• Force
• Pressure
• Torque
• Energy
• Power
• Time
• Temperature
• DynamicViscosity
• KineticViscosity
• Adimensional

Their constructors can take the optional arguments

• value A number. Defaults to NaN. This is stored in the this.value property, provided that unit (see below) is one of the valid quantity units listed in this.units. In other case, this._value is set to NaN. For the builtin quantity classes, this._value attribute coincides with the value of the quantity in the corresponding SI base unit (the first unit in this.units).

• name A string. Defaults to ''. Stored in the this.name attribute. Not used by the library. It is an user field.

• precision A number within 1 and 21 or null. Defaults to 15. Stored in the this.precision attribute. If .precision is a number, all unit queries are strings formatted with 'Number.prototype.toPrecision(quantity.precision)'. If precision is null, raw numbers are returned:

  $ node
  > Length = require('quantity-views').Length
  [Function: Length]
  > distance = new Length()
  Length { _value: NaN, name: '', precision: 15, unit: 'metre' }
  > distance.nauticalMille = 2
  2
  > distance.kilometre
  '3.70400000000000'
  > distance.precision = 5
  5
  > distance.kilometre
  '3.7040'
  > distance.mille
  '2.3016'
  > distance.precision = null
  null
  > distance.kilometre
  3.704
  > distance.mille
  2.301558896047085
  > distance.kilometre = ''
  ''
  > distance.mille
  NaN
  > distance.precision = 10
  10
  > distance.mille
  'NaN'

• unit A string or null. Defaults to null. If it is null and this.units is set, this.unit is set to this.units[0], else this.unit takes this value.

  this.unit has a side effect in this.value:

  $ node
  > Length = require('quantity-views').Length
  [Function: Length]
  > distance = new Length(3)
  Length { _value: 3, name: '', precision: 15, unit: 'metre' }
  > distance.units
  [ 'metre',
    'centimetre',
    'millimetre',
    'inch',
    'foot',
    'footUS',
    'yard',
    'kilometre',
    'nauticalMille',
    'mille',
    'milleUS' ]
  > distance.unit
  'metre'

  The value property returns metres:

    > distance.value
    '3.00000000000000'

  Now, if unit property is set to inches, the value property returns inches:

    > distance.unit = 'inch'
    'inch'
    > distance.value
    '118.110236220472'

  And setting value property is the same as if inches property were set

    > distance.value = 100
    100
    > distance.metre
    '2.54000000000000'
    > distance.inch
    '100.000000000000'
    > distance.value
    '100.000000000000'

  If a invalid value for unit is established, value returns `undefined,

    > distance.unit = 'foo'
    'foo'
    > distance.value
    undefined

  and subsequent assignments to the valueproperty are ignored:

    > distance.value = 1
    1
    > distance.inch
    '100.000000000000'

User defined quantities

Builtin quantity classes are described in lib/defs.js. Feel free of defining your own quantity classes in the same format.

The class Quantity is also exported when requiring quantity-views. It is a constructor for new quantity classes:

$ node
Quantity = require('quantity-views').Quantity
> MyQties = Quantity.createFromDefs({
... mylength: [
... {name: 'astronomicalUnit', multiplier: 1/149597870700, symbol: 'AU'},
... {name: 'lightSecond', multiplier: 1/299792458, symbol: ''},
... {name: 'parsec', multiplier: 1/3.085677581e16, symbol: 'pc'}]})
{ Mylength: [Function: Mylength] }
> coriolanusOdometre = new MyQties.Mylength(66)
Mylength { 
  _value: 9873459466200,
  name: '',
  precision: 15,
  unit: 'astronomicalUnit' }
> coriolanusOdometre.astronomicalUnit
'66.0000000000000'
> coriolanusOdometre.lightSecond
'32934.3157331863'
> coriolanusOdometre.parsec
'0.000319977029583247'
> coriolanusOdometre._value
9873459466200

Note also that in this example the ._value is not stored in any unit of the quantity.

Changelog

• v 1.1.3: Mass units fixed.

• v 1.1.2: Density symbols fixed.

• v 1.1.1: Density symbols fixed.

• v 1.1.0: unit and value properties added.

• v 1.0.0: Initial version

License

This code is subject to MIT license.