predication

Predication uses objects to describe predicates. That way you can store these descriptions as data and easily convert them into pre-compiled functions. This is useful if you need to store complicated predicates in a database or send them as a message, for example.

It supports common predicates and logical operators out of the box, and you can register your own predicates.

There are no external dependencies, and the umd bundle is less than 4kb uglified, and less than 1.5kb gzipped.

installation

npm

$ npm install predication

usage

Predication takes a description and returns a predicate. So it looks like this...

import { predication } from 'predication'

const isTrue = predication({eq: true})

isTrue(true)  // true
isTrue(false) // false

That example is sort of silly, but here is a more complicated description that matches numbers less than 15, and not less than 5, and divisible by either 2 or 3...

import { predication } from 'predication'

const description = {
  and: [
    {lt: 15},
    {not: {lt: 5}},
    {or: [
      {mod: 2},
      {mod: 3}
    ]}
  ]
}

const predicate = predication(description)

const values = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]

const matches = values.filter(predicate) // [6,8,9,10,12,14]

Working with objects

In the likely event that your values are objects, you can use this to ‘key’ into the object...

const predicate = predication({
  this: 'foo',
  eq: true
})

predicate({foo: true}})  // true
predicate({foo: false}}) // false

You can key into the object in more complicated ways. For example...

const predicate = predication({
  this: 'foo.bar[0]',
  eq: true
})

predicate({foo: {bar: [true, false]}}) // true
predicate({foo: {bar: [false, true]}}) // false

You can also key into arrays starting from the end using negative indexing...

const predicate = predication({
  this: 'foo.bar[-0]',
  eq: true
})

predicate({foo: {bar: [true, false]}}) // false
predicate({foo: {bar: [false, true]}}) // true

You can even specify relationships inside the object using that. Here is an example that matches objects whose foo and bar properties are the same...

const predicate = predication({
  this: 'foo',
  eq: {that: 'bar'}
})

predicate({foo: true, bar: true})  // true
predicate({foo: true, bar: false}) // false

Nesting this

The strings provided for this will nest. Take, for example, Bobby and Marian...

const Bobby = {
  body: {
    height: 60,
    age: 33
  }
}

const Marian = {
  body: {
    height: 49,
    age: 72
  }
}

If we wanted want to match people who are either taller than 50" or older than 65, we could do the following...

const tall_or_old = predication({
  this: 'body',                 // <-- we select the body object
  or: [
    { this: 'height', gt: 50 }, // <-- we don't say body.height
    { this: 'age', gt: 45 }     // <-- we don't say body.age
  ]
})

Which would match both Bobby and Marian...

tall_or_old(Bobby)  // true
tall_or_old(Marian) // true

Registering your own predicates

If you want to add support for your own predicates, you can use registerPredicate. The following would add a predicate that returns true when a value has a given root, for example a square root...

import { registerPredicate, predication } from 'predication'

const myHasRootPredicate = (config, value) => (
  (value > 0) && (config !== 0) && (Math.pow(value, (1 / config)) % 1 === 0)
)

registerPredicate('hasRoot', myHasRootPredicate)

Now that you have registered it, you can use hasRoot as a key in your descriptions...

const hasSquareRoot = predication({
  hasRoot: 2
})

hasSquareRoot(4) // true
hasSquareRoot(7) // false
hasSquareRoot(9) // true

Because registerPredicate is used internally, you can take advantage of this and that for your predicates too...

const fooHasCubeRoot = predication({
  this: 'foo',
  hasRoot: 3
})

fooHasCubeRoot({foo: 27}) // true
fooHasCubeRoot({foo: 9})  // false

If you want to validate a configuration, you can pass a predicate as the third argument to register predicate like so...

const greaterThan = (v, config) => v > config
const isNumber = config => typeof config === 'number' // returns false if invalid
registerPredicate('custom_gt', greaterThan, isNumber)

predication({custom_gt: true}) // throws, because true is not a number

Built-in predicates

In the above examples, eq, mod, and lt are examples of built-in predicate names. Here is the full list:

Name	Example	Explanation
eq	{eq: true}	value equals true
ne	{ne: true}	negation of eq
in	{in: [1, 2, 3]}	value is included in array
in	{in: 'abc'}	value includes string 'abc' (case-insensitive)
nin	{nin: ...}	negation of in

registerPredicate('not', not);
registerPredicate('and', and);
registerPredicate('or', or);

registerPredicate('mod', c => v => (Array.isArray(c) ? modR(v, c) : mod(v, c)));

registerPredicate('ne',  c => v => v !== c);
registerPredicate('lt',  c => v => v < c);
registerPredicate('gt',  c => v => v > c);
registerPredicate('lte', c => v => v <= c);
registerPredicate('gte', c => v => v >= c);
registerPredicate('rng', c => v => (v >= c[0] && v <= c[1]));
registerPredicate('oi',  c => v => objectIncludesString(v, c));
registerPredicate('noi', c => v => !objectIncludesString(v, c));

A word about not and missing properties

...

More on 'this' and 'that'

... (how validation handles bad that values)