Trent

Type specifications

npm install trent

This library provides runtime type specifications with a focus on:

• A small, native-feeling syntax for defining specifications
• Good error messages

Usage

import {createSpec} from 'trent'

const Matrix = createSpec(() => [[Number]])

const errors = Matrix.getErrors([
  [1, 2, 3],
  [4, 5, 'wrong']
])

console.log(errors)
/* => [ Error('Value[1][2] must be of type "number"') ] */

Documentation

createSpec(builder: builtInChecks -> Check): Spec

Create a Spec using the builder function which receives the built-in checks described below. The builder must return a valid check or an error throws. The Spec object returned has a method getErrors(value) which returns an array of type errors for value.

Checks

• Value checks
  • Spec
  • Constructor
  • Array [check]
  • Object {key: check, ..., keyN: checkN}
• Built-in checks
  • is(value : any)
  • or(checks : [Check])
  • and(checks : [Check])
  • not(check: Check)
  • maybe(check : Check)
  • tuple(checks : [Check])
  • nullable(check : Check)
  • voidable(check : Check)
• Custom checks
  • Using checkCustomCheck({descriptor: String, isValid: Function})

---

Spec

Check that a provided value matches the Spec.

import {createSpec} from 'trent'
import assert from 'assert'

const Num = createSpec(() => Number)
const Matrix = createSpec(() => [[Num]])

const errors = Matrix.getErrors([[1, 2, 3], [4, 5, 6]])
assert(errors.length === 0)

---

Constructor

Check that a provided value is an instanceof Constructor, or typeof if the constructor is for a primitive such as Number or String.

import {createSpec} from 'trent'
import assert from 'assert'

const Num = createSpec(() => Number)
const errors = Num.getErrors(8)
assert(errors.length === 0)

Note: Any Constructor will work if root[Constructor.name] === Constructor where root is the window in browsers and global in Node.

---

Array [check]

Check that a provided value is an array where every element passes the check.

import {createSpec} from 'trent'
import assert from 'assert'

const Numbers = createSpec(() => [Number])
const errors = Numbers.getErrors([8])
assert(errors.length === 0)

---

Object {key: check, ..., keyN: checkN}

Check that a provided value is an object where every key value passes its check.

import {createSpec} from 'trent'
import assert from 'assert'

const Point = createSpec(() => ({x: Number, y: Number}))
const errors = Point.getErrors({x: 8, y: 8})
assert(errors.length === 0)

---

is(value : any)

Check that a provided value must strictly equal (===) the value.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({is}) => is(8))
const errors = Eight.getErrors(8)
assert(errors.length === 0)

---

or(checks : [Check])

Check that a provided value matches at least one of the checks.

import {createSpec} from 'trent'
import assert from 'assert'

const NumberOrString = createSpec(({or}) => or([Number, String]))
const errors = NumberOrString.getErrors(8)
assert(errors.length === 0)

---

and(checks : [Check])

Check that a provided value matches at every one of the checks.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({and, is}) => and([Number, is(8)]))
const errors = Eight.getErrors(8)
assert(errors.length === 0)

---

not(check : Check)

Check that a provided value does not pass the check.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({not, is}) => not(is(9)))
const errors = Eight.getErrors(8)
assert(errors.length === 0)

---

maybe(check : Check)

Check that a provided value matches check or is null or undefined.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({maybe, is}) => maybe(is(8)))
assert(Eight.getErrors(8).length === 0)
assert(Eight.getErrors(null).length === 0)
assert(Eight.getErrors(undefined).length === 0)

---

tuple(checks : [Check])

Check that a provided value is an array with length equal to checks.length and the first element passes the first check and so on.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({tuple, is}) => tuple([is(8), is(8)]))
assert(Eight.getErrors([8, 8]).length === 0)

---

nullable(check : Check)

Check that a provided value matches check or is null.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({nullable, is}) => nullable(is(8)))
assert(Eight.getErrors(8).length === 0)
assert(Eight.getErrors(null).length === 0)

---

voidable(check : Check)

Check that a provided value matches check or is undefined.

import {createSpec} from 'trent'
import assert from 'assert'

const Eight = createSpec(({voidable, is}) => voidable(is(8)))
assert(Eight.getErrors(8).length === 0)
assert(Eight.getErrors(undefined).length === 0)

---

Using checkCustomCheck({descriptor: String, isValid: Function})

Create a check where descriptor fits the sentence {subject} must be {descriptor} for error messages and isValid(value: any) returns whether the value passes the check.

import {createSpec, createCustomCheck} from 'trent'
import assert from 'assert'

const lowercase = createCustomCheck({
  descriptor: 'lowercase',
  isValid (x) {
    return typeof x === 'string' && x.toLowerCase() === x
  }
})

const Code = createSpec(() => lowercase)
assert(Code.getErrors('8').length === 0)

Note: Custom checks cannot nest checks within them. In a tree structure analogy, custom checks must be leaves. The reason for this limitation is the complexity of creating good error messages.

createDependentSpecs(builder : builtInChecks -> {Check}) {Spec}

Create a collection of Specs which are dependent and/or recursive.

For example, we have two types Foo and Bar which both can contain each other. We can try to write this system with createSpec:

import {createSpec} from 'trent'
var Foo = createSpec(() => ({barList: [Bar]}))
var Bar = createSpec(() => ({fooList: [Foo]}))

Creating Foo with undefined Bar will not work. We need createDependentSpecs to enable a "late-binding" where order does not matter. We use the ref built-in check available to createDependentSpecs to reference Specs.

import {createDependentSpecs} from 'trent'
import assert from 'assert'
const {Foo} = createDependentSpecs(({ref}) => ({
  Foo: {barList: [ref('Bar')]},
  Bar: {fooList: [ref('Foo')]}
}))

assert(Foo.getErrors({
  barList: [{
    fooList: [{
      barList: [{
        fooList: []
      }]
    }]
  }]
}).length === 0)

Real-life example

I maintain the HTML parser Himalaya which follows a strict specification for its output. The output contains Nodes which can have children Nodes, so we need a recursive type.

import {
  createSpec,
  createDependentSpecs
} from 'trent'

// I pull this out to show that you can
const Text = createSpec(({is}) => ({
  type: is('text'),
  content: String
}))

export const {Node} = createDependentSpecs(({is, or, ref, nullable}) => ({
  Node: or([
    ref('Element'),
    ref('Comment'),
    Text
  ]),
  Element: {
    type: is('element'),
    tagName: String,
    children: [ref('Node')],
    attributes: [{
      key: String,
      value: nullable(String)
    }]
  },
  Comment: {
    type: is('comment'),
    content: String
  }
}))