Unitmath NPM | npm.io

UnitMath

UnitMath is a JavaScript library for unit conversion and arithmetic.

Demo

https://ericman314.github.io/UnitMath/

Install

npm install unitmath

Getting Started

const unit = require('unitmath')

let a = unit('5 m').div('2 s')   // 2.5 m / s
let b = unit('40 km').to('mile')  // 24.8548476894934 mile
b.toString({ precision: 4 })          // "24.85 mile"

Creating Units

To create a unit, call unit with either a single string, or a number and a string:

// String
let a = unit('40 mile')
let b = unit('hour')

// Number and string
let g = unit(9.8, 'm/s^2')
let h = unit(19.6, 'm')

Units can be simple (4 kg) or compound (8.314 J/mol K). They may also be valueless (hour).

Parsing Rules

Parsing rules are different than typical JavaScript syntax. For instance:

The string may only contain one "/". Units appearing before the / will be in the numerator of the resulting unit, and units appearing after the / will be in the denominator.
Parentheses () and asterisk * are ignored: multiplication is implicit.
Extra whitespace is allowed.

The following all form valid units:

// These are valid:
unit("2 in")
unit("60/s")
unit("8.314 * kg * m^2 / (s^2 * mol * K)") 
unit("6.022e-23 mol^-1")
unit("kW / kg K")
unit("3.1415926535897932384626433832795 rad") 

// These are invalid:
unit("2 in + 3 in") // Unexpected "+" (Use .add() instead)
unit("60 / s / s") // Unexpected additional "/"
unit("0x123 kg") // Unit "x123" not found

The exact syntax is as follows:

[value][numerator][/denominator]

numerator, denominator:
atomicUnit [atomicUnit ...]

atomicUnit:
[prefix]unit[^power]

value, power:
Any floating-point number

unit, prefix:
Any built-in or user-defined unit or prefix

Unit Conversion

The to method converts from one unit to another. The two units must have the same dimension.

// Convert units
unit('40 km').to('mile') // 24.8548476894934 mile
unit('kg').to('lbm') // 2.20462262184878 lbm
unit(5000, 'kg').to('N s') // Cannot convert 5000 kg to N s: dimensions do not match

The split method will convert one unit into an array of units like so:

// Split a unit into parts
unit('10 km').split([ 'mi', 'ft', 'in' ]) // [ 6 mi, 1128 ft, 4.78740157486361 in ]
unit('51.4934 deg').split([ 'deg', 'arcmin', 'arcsec' ]) // [ 51 deg, 29 arcmin, 36.24 arcsec ]

Arithmetic

Use the methods add, sub, mul, div, pow, sqrt, and others to perform arithmetic on units. Multiple operations can be chained together:

// Chain operations together
let g = unit(9.8, 'm/s^2')
let h = unit(19.6, 'm')
h.mul(2).div(g).sqrt()   // 2 s

Strings and numbers are implicitly converted to units within a chained expression. When chaining operators, they will execute in order from left to right. This may not be the usual, mathematical order of operations:

// Strings are implicitly converted to units
unit('3 ft').add('6 in').mul(2)   // 7 ft

All of the operators are also available on the top-level unit object as static functions:

// Using static functions
unit.mul(unit.add('3 ft', '6 in'), 2)   // 7 ft

Units are immutable, so every operation on a unit creates a new unit.

Simplify

UnitMath has a simplify method that will attempt to simplify a unit:

// Simplify units
unit('10 / s').simplify() // 10 Hz
unit('J / m').simplify() // N

Because units are immutable, simplify always returns a new unit with the simplified units. It does not modify the original unit.

In the simplify method, simplification is performed in two steps: 1. Attempting to simplify a compound unit into a single unit in the desired unit system 2. Choosing an appropriate prefix (if options.autoPrefix is true)

Note: To choose the best prefix without automatically simplifying the Unit, use the applyBestPrefix method.

Here are the options available for the simplify method:

system - Default: 'auto'. The unit system to use when simplifying a unit. Available systems are si, cgs, us, and auto.

// Simplify units using a specific system
unit('8 kg m / s^2').simplify({ system: 'si' }) // 8 N
unit('8 kg m / s^2').simplify({ system: 'cgs' }) // 800 kdyn
unit('8 kg m / s^2').simplify({ system: 'us' }) // 1.79847154479768 lbf

When system is 'auto' (the default), UnitMath will try to infer the unit system from the individual units that make up that unit. This is not guaranteed to succeed in all cases, so whenever possible, the system should be specified explicitly.

unit = unit.config({ system: 'auto' })

// Automatically infer the unit system while simplifying
unit('150 lbf').div('10 in^2').simplify()  // 15 psi
unit('400 N').div('10 cm^2').simplify()  // 400 kPa

Specifying a unit system other than 'auto' will force UnitMath to use the specified system. Use the config function to apply the system everywhere, or pass a configuration object to simplify to apply to a single statement:

unit = unit.config({ system: 'us' })

let a = unit('5 m').div('2 s')
a.simplify() // 8.202099737532809 ft / s

// Override the system for this statement
a.simplify({ system: 'si'}) // 2.5 m / s

autoPrefix - Default: true. This option specifies whether simplify will try to choose an appropriate prefix in case of very small or very large quantities.
```
unit('1e-10 kg m / s^2').simplify() // 0.0001 uN
unit('1e-10 kg m / s^2').simplify({ autoPrefix: false }) // 1e-10 N
```
prefixMin and prefixMax - Defaults: 0.1 and 1000. Sets the threshold for choosing a different prefix when autoPrefix is true.
```
unit('0.005 m').simplify() // 0.5 cm
unit('0.005 m').simplify({ prefixMin: 0.001 }) // 0.005 m
```
formatPrefixDefault - Default: none. By default, certain units are never assigned a new prefix (this is controlled by the formatPrefixes property of the unit definition.) By setting formatPrefixDefault to "all", this behavior can be changed.
```
unit('1000001 lumen').simplify() // 1000001 lumen
unit('1000001 lumen').simplify({ formatPrefixDefault: 'all' }) // 1.000001 megalumen
```

Formatting

Use toString to output a unit as a string:

unit('1 lb').to('kg').toString() // "0.45359237 kg"

The toString method accepts a configuration object. The following options are available:

precision - Default: 15. The number of significant figures to output when converting a unit to a string. Reducing this can help reduce the appearance of round-off errors. Setting a value of 0 will disable rounding.
```
// Output a unit with a specific precision
unit('180 deg').to('rad').toString({ precision: 6 }) // 3.14159 rad
unit('1 lb').to('kg').toString({ precision: 4 }) // "0.4536 kg"
```
parentheses - Default: false. When formatting a unit, group the numerator and/or denominator in parentheses if multiple units are present. This is useful when the resulting string will be used in a mathematical expression.
```
// Output a unit with parentheses
unit('45 W / m K').toString({ parentheses: true }) // 45 W / (m K)
```

formatter. Define a custom formatter for the numeric portion of the unit. The formatter will be passed the numeric value of the unit. For example:

let unit = require('unitmath')

// Custom formatter
let formatter = new Intl.NumberFormat('en-US', {
  style: 'currency',
  currency: 'USD',
});

// Specify formatter in argument to toString
unit('25000 / ton').toString({ formatter: formatter.format }) // "$25,000.00 ton^-1"

// Specify formatter in config
let unitMoney = unit.config({
  formatter: formatter.format
})
unitMoney('25000 / ton').toString() // "$25,000.00 ton^-1"

The toString method outputs a unit exactly as it is represented internally. It does not automatically simplify a unit. This means you will usually want to call simplify before calling toString. How and when to simplify a unit is very subjective. UnitMath cannot anticipate all needs, so the user must explicitly call simplify when needed.

// toString outputs units "as-is".
unit('10 / s').toString() // "10 s^-1"
unit('10 / s').simplify().toString() // "10 Hz"

Configuring

UnitMath can be configured using unit.config(options). The function returns a new instance of UnitMath with the specified configuration options.

// Set the default unit system to "us"
const unit = require('unitmath').config({ system: 'us' })

Available options are:

Any of the simplify options.
Any of the formatting options.
type. An object that allows UnitMath to work with custom numeric types. See Custom Types for complete details and examples.

definitions. An object that allows you to add to, modify, or remove the built-in units. See User-Defined Units for complete details.

// Create a new configuration
unit = unit.config({
  definitions: {
    units: {
      furlong: { value: '220 yards' },
      fortnight: { value: '2 weeks' }
    }
  },
  precision: 6
})

unit('6 furlong/fortnight').to('m/s') // 0.000997857 m / s

Because unit.config(options) returns a new instance of UnitMath, is is technically possible to perform operations between units created from different instances. The resulting behavior is undefined, however, so it is probably best to avoid doing this.

Important: unit.config(options) returns a new instance of the factory function, so you must assign the return value of unit.config(options) to some variable, otherwise the new options won't take effect:

let unit = require('unitmath')

// Incorrect, has no effect!
unit.config(options)

// Correct
unit = unit.config(options)

Querying the current configuration

Call unit.getConfig() to return the current configuration.

// Get the current configuration
unit.getConfig() // { system: 'si', ... }

Extending UnitMath

User-Defined Units

To create a user-defined unit, pass an object with a definitions property to unit.config():

// Create a new configuration adding a user-defined unit
unit = unit.config({
  definitions: {
    units: {
      lightyear: { value: '9460730472580800 m' }
    }
  }
})

unit('1 lightyear').to('mile') // 5878625373183.61 mile

The definitions object contains four properties which allow additional customization of the unit system: units, prefixGroups, systems, and skipBuiltIns.

definitions.units

This object contains the units that are made available by UnitMath. Each key in definitions.units becomes a new unit. The easiest way to define a unit is to provide a string representation in terms of other units:

// Define new units in terms of existing ones
definitions: {
  units: {
    minute: { value: '60 seconds' },
    newton: { value: '1 kg m/s^2' }
  }
}

Here are all the options you can specify:

value: (Required) The value of the unit. It can be a string or an array containing two items:

// value can be a string...
definitions: {
  units: {
    minute: { value: '60 seconds' },
    newton: { value: '1 kg m/s^2' }
  }
}

// ...or an array.
definitions: {
  units: {
    minute: { value: [ 60, 'seconds' ] },
    newton: { value: [ 1, 'kg m/s^2' ] }
  }
}

quantity: Specifying a quantity will create a new base unit. This is required for units that are not defined in terms of other units, such as meter and second. In this case, value will just be a number (or custom type):

// Specify `quantity` creates a new base unit
definitions: {
  units: {
    seconds: { quantity: 'TIME', value: 1 }
  }
}

Only use quantity to define base units. Do not use quantity to define a derived unit:

// Incorrect
definitions: {
  units: {
    meter: { quantity: 'LENGTH', value: 1 },
    squareMeter: { quantity: 'LENGTH^2', value: 1 }
  }
}

// Correct
definitions: {
  units: {
    meter: { quantity: 'LENGTH', value: 1 },
    squareMeter: { value: '1 meter^2' }  
  }
}

prefixGroup - Default: 'NONE'. Specifies which prefix group will be used when parsing and formatting the unit.

definitions: {
  units: {
    // Will parse 'nanometer', 'micrometer', 'millimeter', 'kilometer', 'megameter', etc.
    meter: { prefixGroup: 'LONG', ... },

    // Will parse 'nm', 'um', 'mm', 'km', 'Mm', etc.
    m: { prefixGroup: 'SHORT', ... }
  }
}

formatPrefixes: A string array that specifies individual items of the unit's prefix group which will be used when formatting a unit. If this option is omitted, the global option formatPrefixDefault determines whether the unit will be formatted using all prefixes in the prefix group, or none at all.

definitions: {
  units: {
    L: {
      value: '1e-3 m^3'
      // Parse any prefix in the 'SHORT' prefix group
      prefixGroup: 'SHORT',
      // Format only as 'nL', 'uL', 'mL', and 'L'.
      formatPrefixes: ['n', 'u', 'm', ''],
    },
    lumen: {
      value: '1 cd sr'
      // Parse any prefix in the 'LONG' prefix group
      prefixGroup: 'LONG',
      // formatPrefixes is not given, so lumen will be formatted 
      // only as "lumen" if formatPrefixDefault === false,
      // or formatted using any of the prefixes in the 'LONG' 
      // prefix group if formatPrefixDefault === true.
    }
  }
}

basePrefix: Optional. The prefix to use for a base unit, if the base unit has one. This is necessary for units such as kilogram, which is a base unit but has a prefix.

// Special case for base units with prefixes, such as kg
definitions: {
  units: {
    g: {
      quantity: 'MASS',
      prefixGroup: 'SHORT',
      formatPrefixes: ['n', 'u', 'm', '', 'k'],
      value: 0.001,
      // Treat as if 'kg' is the base unit, not 'g'
      basePrefix: 'k'
    }
  }
}

aliases: Shortcut to create additional units with identical definitions.

// Create aliases for a unit
definitions: {
  units: {
    meter: { ... , aliases: [ 'meters' ] }
  }
}

offset - Default: 0: Used when the zero-value of this unit is different from the zero-value of the base unit.

// Special case for units with an offset from the base unit, like celsius
definitions: {
  units: {
    celsius: {
      value: '1 K',
      offset: 273.15
    }
  }
}

definitions.prefixGroups

The definitions.prefixGroups object is used to define strings and associated multipliers that are prefixed to units to change their value. For example, the 'k' prefix in km multiplies the value of the m unit by 1000.

For example:

// Define prefix groups
definitions: {
  prefixGroups: {
    NONE: { '': 1 },
    SHORT: {
      m: 0.001,
      '': 1,
      k: 1000
    },
    LONG: {
      milli: 0.001,
      '': 1,
      kilo: 1000
    }
  }
}

definitions.systems

This object assigns one or more units to a number of systems. Each key in definitions.systems becomes a system. For each system, list all the units that should be associated with that system in an array. The units may be single or compound (m or m/s) and may include prefixes.

Example:

// Define unit systems
definitions: {
  systems: {
    si: ['m', 'kg', 's', 'N', 'J', 'm^3', 'm/s'],
    cgs: ['cm', 'g', 's', 'dyn', 'erg', 'cm^3', 'cm/s'],
    us: ['ft', 'lbm', 's', 'lbf', 'btu', 'gal', 'ft/s']
  }
}

When UnitMath formats a unit, it will try to use one of the units from the specified system.

definitions.skipBuiltIns

A boolean value indicating whether to skip the built-in units. If true, only the user-defined units, prefix groups, and systems that are explicitly included in definitions will be created.

// Skip built-in units: only the user-defined units,
// prefix groups, and systems will be created
definitions: {
  skipBuiltIns: true
}

Querying current unit definitions

You can view all the current definitions by calling unit.definitions(). This object contains all the units, prefix groups, and systems that you have configured, including the built-ins (unless definitions.skipBuiltIns is true).

unit.definitions()

Below is an abbreviated sample output from unit.definitions(). It can serve as a starting point to create your own definitions.

// Sample `definitions` config
{ 
  units: {
    '': { quantity: 'UNITLESS', value: 1 },
    meter: {
      quantity: 'LENGTH',
      prefixGroup: 'LONG',
      formatPrefixes: [ 'nano', 'micro', 'milli', 'centi', '', 'kilo' ],
      value: 1,
      aliases: [ 'meters' ]
    },
    m: {
      prefixGroup: 'SHORT',
      formatPrefixes: [ 'n', 'u', 'm', 'c', '', 'k' ],
      value: '1 meter'
    },
    inch: { value: '0.0254 meter', aliases: [ 'inches', 'in' ] },
    foot: { value: '12 inch', aliases: [ 'ft', 'feet' ] },
    yard: { value: '3 foot', aliases: [ 'yd', 'yards' ] },
    mile: { value: '5280 ft', aliases: [ 'mi', 'miles' ] },
    ...
  },
  prefixGroups: {
    NONE: { '': 1 },
    SHORT: {
      '': 1,
      da: 10,
      h: 100,
      k: 1000,
      ...
      d: 0.1,
      c: 0.01,
      m: 0.001,
      ... 
    },
  },
  systems: {
    si: ['m', 'meter', 's', 'A', 'kg', ...],
    cgs: ['cm', 's', 'A', 'g', 'K', ...],
    us: ['ft', 's', 'A', 'lbm', 'degF', ...]
  }
}

Custom Types

You can extend UnitMath to work with custom types. The type option is an object containing several properties, where each property value is a function that replaces the normal +, -, *, /, and other arithmetic operators used internally by UnitMath.

Example using Decimal.js as the custom type:

// Configure UnitMath to use Decimal.js
const Decimal = require('decimal.js')
const unit = unit.config({
  type: {
    clone: (x) => new Decimal(x),
    conv: (x) => new Decimal(x),
    add: (a, b) => a.add(b),
    sub: (a, b) => a.sub(b),
    mul: (a, b) => a.mul(b),
    div: (a, b) => a.div(b),
    pow: (a, b) => a.pow(b),
    eq: (a, b) => a.eq(b),
    lt: (a, b) => a.lt(b),
    le: (a, b) => a.lte(b),
    gt: (a, b) => a.gt(b),
    ge: (a, b) => a.gte(b),
    abs: (a) => a.abs(),
    round: (a) => a.round(),
    trunc: (a) => Decimal.trunc(a)
  }
})

let u = unit2('2.74518864784926316174649567946 m')

Below is a table of functions, their description, and when they are required:

Function	Description	Required?
`clone: (a: T) => T`	Create a new instance of the custom type.	Always
`conv: (a: number \\| string \\| T) => T`	Convert a number or string into the custom type.	Always
`add: (a: T, b: T) => T`	Add two custom types.	Always
`sub: (a: T, b: T) => T`	Subtract two custom types.	Always
`mul: (a: T, b: T) => T`	Multiply two custom types.	Always
`div: (a: T, b: T) => T`	Divide two custom types.	Always
`pow: (a: T, b: number) => T`	Raise a custom type to a power.	Always
`abs: (a: T) => T`	Return the absolute value of a custom type.	For autoPrefix: true
`lt: (a: T, b: T) => boolean`	Compare two custom types for less than.	For autoPrefix: true
`le: (a: T, b: T) => boolean`	Compare two custom types for less than or equal.	For autoPrefix: true
`gt: (a: T, b: T) => boolean`	Compare two custom types for greater than.	For autoPrefix: true
`ge: (a: T, b: T) => boolean`	Compare two custom types for greater than or equal.	For autoPrefix: true
`eq: (a: T, b: T) => boolean`	Compare two custom types for equality.	For the `equals` function
`round: (a: T) => T`	Round a custom type to the nearest integer.	For the `split` function
`trunc: (a: T) => T`	Truncate a custom type to the nearest integer.	For the `split` function

The add, sub, mul, div, and pow functions replace +, -, *, /, and Math.pow, respectively. The clone function should return a clone of your custom type (same value, different object).

The conv function must, at a minimum, be capable of converting both strings and numbers into your custom type. If given a custom type, it should return it unchanged, or return a clone. Among other things, the conv function is used by UnitMath to convert the values of the built-in units to your custom type during initialization.

UnitMath will also use the conv function when constructing units from numbers and strings. If your custom type is representable using decimal or scientific notation (such as 6.022e+23), you can include both the value and the units in a single string:

// Supply a single string, and the numeric portion will be parsed using type.conv
unit('3.1415926535897932384626433832795 rad')

If your custom type cannot be represented in decimal or scientific notation, such as is the case with complex numbers and fractions, you will have to pass your custom type and the unit string separately:

unit(Fraction(1, 2), 'kg')

The functions clone, conv, add, sub, mul, div, and pow are always required. Omitting any of these will cause the config method to throw an error. The other functions are conditionally required, and you will receive an error if you attempt something that depends on a function you haven't provided.

API Reference

In the function signatures below, the T type is the custom type you have provided, or number if you have not provided a custom type.

Constructor

unit(value: T, unitString: string): unit
unit(value: T): unit
unit(valueAndUnitString: string): unit
unit(): unit

Creates a unit with the specified value and unit string. If valueAndUnitString is supplied, it must specify both the numeric portion and the units portion of the unit.

const unit = require('unitmath')
unit(60, 'mile/hour') // 60 mile / hour
unit(60) // 60
unit('60 mile/hour') // 60 mile / hour
unit('mile/hour') // mile / hour
unit() // Empty unit

unit() will parse the special strings "NaN", "Infinity", and "-Infinity" as the corresponding numeric values:

const unit = require('unitmath')
unit('Infinity kg') // Infinity kg

The string used to specify the unit (valueAndUnitString or unitString) must be in the following format:

[value][numerator][/denominator]

numerator, denominator:
atomicUnit [atomicUnit ...]

atomicUnit:
[prefix]unit[^power]

value:
Any floating-point number, or the strings "NaN", "Infinity", or "-Infinity" (without the quotes).

power:
Any floating-point number

Member Functions

add(other: unit | string | T) : unit
Adds another unit to this unit. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.
```
let a = unit('20 kW')
let b = unit('300 W')
let sum = a.add(b) // 20.3 kW
```
sub(other: unit | string | T) : unit
Subtract another unit from this unit. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.
```
let a = unit('4 ft')
let b = unit('1 yd')
let difference = a.sub(b) // 1 ft
```
mul(other: unit | string | T) : unit
Multiplies this unit by another unit. If a string or number is supplied as an argument, it is converted to a unit.
```
let a = unit('8 m')
let b = unit('200 N')
let product = a.mul(b).simplify() // 1.6 kJ
```
div(other: unit | string | T) : unit
Divides this unit by another unit. If a string or number is supplied as an argument, it is converted to a unit.
```
let a = unit('64 kJ')
let b = unit('16 s')
let quotient = a.div(b).simplify() // 4 kW
```
pow(p: number)
Raises this unit to the power p and returns a new unit.
```
let result = unit('10 m').pow(3) // 1000 m^3
```

sqrt()

Returns the square root of this unit.

unit('1 hectare').sqrt().simplify() // 100 m

abs()
Returns the absolute value of this unit. If this unit has an offset, such as degC, this is applied before taking the absolute value.
```
unit('-5 m / s').abs() // 5 m / s
unit('-293.15 degC').abs() // -253.15 degC
```

clone()

Clones this unit.

let unit = require('unitmath')

let a = unit('40 m/s') // 40 m / s
let b = a.clone() // 40 m / s

to(target: unit | string)
Converts this unit to the specified target unit or string. The returned unit will be "fixed", so it will not be auto-simplified or auto-prefixed in format().
```
let r = unit('10 kg m^2 / s^3 A^2')
r.simplify().toString() // 10 ohm
r.to('kohm').toString() // 0.01 kohm
```
toBaseUnits()
Returns a new unit in the base representation.
```
unit('10 ft/s').toBaseUnits() // 3.048 m / s
```

getValue()

Returns the value of this unit, or null if the unit is valueless.

unit('70 mile/hour').getValue() // 70
unit('km / hour').getValue() // null

setValue(x: number | string | custom)
Returns a copy of this unit but with its value replaced with the given value. Useful if you would like to perform your own operations on a unit's value. If supplied with no arguments, or null, will remove the value from the unit.
```
unit('10 m').setValue(20) // 20 m
unit('m').setValue(20) // 20 m
unit('10 ft').setValue(20) // 20 ft
unit('10 ft').setValue() // ft
```
getNormalizedValue()
Returns the value of this unit if it were to be converted to SI base units (or whatever base units that are defined). Returns null if the unit is valueless.
```
unit('10 ft/s').getNormalizedValue() // 3.048
```
setNormalizedValue()
Returns a copy of this unit but with its value replaced with the given normalized value.
```
unit('ft / s').setNormalizedValue(3.048) // 10 ft / s
```
simplify()
Attempts to simplify this unit, and returns the simplified unit (or a clone of the original if unsuccessful). simplify() is called when a unit is being formatted as a string whenever the config option simplify is 'auto' or 'always'.
```
unit('10 N m').simplify() // 10 J
```
applyBestPrefix(PrefixOptions)
Chooses a prefix for this unit so that its value is between prefixMin and prefixMax, and returns a new unit with the prefix applied. If the value is already within the range, the unit is returned unchanged. The prefix is chosen from the unit's formatPrefixes property.
```
unit('0.0001 m').applyBestPrefix() // 0.1 mm
unit('0.1 m').applyBestPrefix() // 10 cm
unit('10000 m').applyBestPrefix() // 10 km
```
split(Array(string | unit))
Returns an array of units that result from splitting this unit into the given units. The sum of the resulting units is equal to this unit, and each of the returned units is the result of truncating this unit to an integer, and then passing the remainder to the next unit, until the final unit, which takes up all the remainder.
```
unit('51.4934 deg').split([ 'deg', 'arcmin', 'arcsec' ]) // [ 51 deg, 29 arcmin, 36.24 arcsec ]
```
getUnits()
Returns a clone of this unit with the value removed. Equivalent to unit.setValue(null).
```
unit('8.314 J / mol K').getUnits() // J / mol K
```

isCompound()

Returns true if this unit's unit list contains two or more units, or one unit with a power not equal to 1.

unit('34 kg').isCompound() // false
unit('34 kg/s').isCompound() // true
unit('34 kg^2').isCompound() // true
unit('34 N').isCompound() // false
unit('34 kg m / s^2').isCompound() // true

isBase()

Returns true if this unit's unit list contains exactly one unit with a power equal to 1, and which is the of same dimension as one of the base dimensions length, time, mass, etc., or a user-defined base dimension.

unit = unit.config({ 
   definitions: {
    units: {
      myUnit: { quantity: 'MY_NEW_BASE', value: 1 },
      anotherUnit: { value: '4 myUnit' }
    }
  }
})

unit('34 kg').isBase() // true
unit('34 kg/s').isBase() // false
unit('34 kg^2').isBase() // false
unit('34 N').isBase() // false
unit('34 myUnit').isBase() // true
unit('34 anotherUnit').isBase() // true

getInferredSystem()

Examines this unit's unitList to determine the most likely system this unit is expressed in.

unit('10 N m').getInferredSystem() // 'si'
unit('10 J / m').getInferredSystem() // 'si'
unit('10 m^3 Pa').getInferredSystem() // 'si'
unit('10 dyne/cm').getInferredSystem() // 'cgs'
unit('10 ft/s').getInferredSystem() // 'us'
unit('10').getInferredSystem() // null

equalsQuantity(other: unit | string)
Returns true if this unit and another unit have equal quantities or dimensions.
```
unit('5 m/s^2').equalsQuantity('4 ft/s^2') // true
```
equals(other: unit | string)
Returns true if the two units represent the same values.
```
unit('3 ft').equals('1 yard') // true
```
compare(other: unit | string)
Returns a value indicating whether this unit is less than (-1), greater than (1), or equal to (0), another unit.
```
unit('30 min').compare('1 hour') // -1
unit('60 min').compare('1 hour') // 0
unit('90 min').compare('1 hour') // 1
```

lessThan(other: unit | string)

Compares this and another unit and returns true if this unit is less than the other.

unit('80 cm').lessThan('1 m') // true
unit('100 cm').lessThan('1 m') // false
unit('120 cm').lessThan('1 m') // false

lessThanOrEqual(other: unit | string)

Compares this and another unit and returns true if this unit is less than or equal to the other.

unit('80 cm').lessThanOrEqual('1 m') // true
unit('100 cm').lessThanOrEqual('1 m') // true
unit('120 cm').lessThanOrEqual('1 m') // false

greaterThan(other: unit | string)

Compares this and another unit and returns true if this unit is greater than the other.

unit('80 cm').greaterThan('1 m') // false
unit('100 cm').greaterThan('1 m') // false
unit('120 cm').greaterThan('1 m') // true

greaterThanOrEqual(other: unit | string)

Compares this and another unit and returns true if this unit is greater than or equal to the other.

unit('80 cm').greaterThanOrEqual('1 m') // false
unit('100 cm').greaterThanOrEqual('1 m') // true
unit('120 cm').greaterThanOrEqual('1 m') // true

toString(options)
Formats this unit as a string. Formatting options can be supplied which will override the configured options. See Formatting for a list of all options and their effects.

Static Functions

add(a: unit | string | T, b: unit | string | T) : unit
Adds two units. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.
```
let sum = unit.add('20 kW', '300 W') // 20.3 kW
```
sub(a: unit | string | T, b: unit | string | T) : unit
Subtract two units. If a string or number is supplied as an argument, it is converted to a unit. Both units must have values and have matching dimensions.
```
let difference = unit.sub('4 ft', '1 yd') // 1 ft
```
mul(a: unit | string | T, b: unit | string | T) : unit
Multiplies two units. If a string or number is supplied as an argument, it is converted to a unit.
```
let product = unit.mul('8 m/s', '200 N').simplify() // 1.6 kW
```
div(a: unit | string | T, b: unit | string | T) : unit
Divides two units. If a string or number is supplied as an argument, it is converted to a unit.
```
let quotient = unit.div('64 kJ', '16 s').simplify() // 4 kW
```
pow(a: unit | string | T, p: number) : unit
Raises a unit to the power p and returns a new unit.
```
let result = unit.pow('10 m', 3) // 1000 m^3
```
sqrt(a: unit | string | T) : unit
Returns the square root of a unit.
```
unit.sqrt('1 hectare').simplify() // 100 m
```
abs(a: unit | string | T) : unit
Returns the absolute value of a unit. If the unit has an offset, such as degC, this is applied before taking the absolute value.
```
unit.abs('-5 m / s') // 5 m / s
unit.abs('300 degC') // -246.3 degC
```
to(a: unit | string | number, b: unit | string)
Converts a unit to the specified target unit or string. The returned unit will be "fixed", so it will not be auto-simplified or auto-prefixed in format().
```
unit.to('10 kg m^2 / s^3 A^2', 'kohm') // 0.01 kohm
```
config(options:object)
Configure a new unit namespace with the given options (see Configuring)
```
const unit = require('unitmath').config({ option1, option2, ... })
```

getConfig()

Returns the current configuration.

const unit = require('unitmath')
unit.getConfig()

exists(singleUnitString:string)
Tests if the given unit, optionally with a prefix, exists.
```
const unit = require('unitmath')
unit.exists('km') // true
```
definitions()
Return the current unit definitions in effect. (User's own definitions can be queried through unit.getConfig().definitions.)

Contributing

This is a community-supported project; all contributions are welcome. Please open an issue or submit a pull request.

Acknowledgements

Many thanks to Jos de Jong (@josdejong), the original author of Unit.js, who suggested the idea of splitting the file off from Math.js and into its own library.