tcomb

What is tcomb?

The tcomb npm package is a library for Node.js and the browser which allows you to check the types of JavaScript values at runtime with a simple and concise syntax. It is useful for enforcing type safety, defining domain models, and creating composable and reusable type checkers.

What are tcomb's main functionalities?

Type Checking

This feature allows you to define a structure with specific types and create instances that adhere to those types.

const t = require('tcomb');
const Person = t.struct({ name: t.String, age: t.Number });
const person = Person({ name: 'Giulio', age: 43 });

Function Argument Validation

This feature enables you to validate function arguments to ensure they are of the expected type.

const t = require('tcomb');
function sum(a, b) {
  t.Number(a);
  t.Number(b);
  return a + b;
}
sum(1, 2);

Subtyping

This feature allows you to create subtypes based on existing types with additional constraints.

const t = require('tcomb');
const Positive = t.subtype(t.Number, n => n >= 0);
const positiveNumber = Positive(10);

Refinement

This feature is similar to subtyping but is specifically for refining types to a more specific subset.

const t = require('tcomb');
const Integer = t.refinement(t.Number, n => n % 1 === 0);
const integerNumber = Integer(42);

Enums

This feature allows you to define a set of constants and ensure values match one of those constants.

const t = require('tcomb');
const Role = t.enums({ Admin: 'Admin', User: 'User' });
const userRole = Role('User');

Other packages similar to tcomb

prop-types

Prop-types is a library for React that is used to document the intended types of properties passed to components. It is similar to tcomb in that it provides runtime type checking, but it is specifically tailored for React's component props.

io-ts

io-ts is a TypeScript runtime type system for IO decoding/encoding. It is similar to tcomb in providing runtime type checking and validation, but it leverages TypeScript's type system for added compile-time safety.

joi

Joi is an object schema description language and validator for JavaScript objects. It provides a powerful and expressive way to define and validate data structures. It is similar to tcomb in terms of validation capabilities but has a different API and is often used for validating API input.

ajv

Ajv is a JSON schema validator. It validates data against JSON Schema standards and is similar to tcomb in terms of validation. However, it uses a JSON-based schema definition rather than a programmatic API.

README

tcomb

JavaScript types and combinators

Why. tcomb is a library which allows you to check the types of JavaScript values at runtime with a simple syntax. It is great for checking external input, for testing and for adding safety to your internal code. Bonus points:

• easy debugging
• encode/decode of your domain objects to/from JSON for free
• instances are immutables by default

How. This library provides several type combinators and a built-in assert function you can use. When an assertion fails this library starts the debugger so you can inspect the stack and quickly find out what's wrong. The debugger starts only once after the first failed assert.

What. You can define and check:

• JavaScript native types
• structs (i.e. classes)
• unions
• maybe
• enums
• tuples
• subtypes
• lists
• function types (experimental)

Quick example

Let's build a product

var Product = struct({
    name: Str,                  // a REQUIRED string
    description: maybe(Str),    // an OPTIONAL string (can be `null`)
    homepage: Url,              // a SUBTYPE of string representing an URL
    shippings: list(Shipping),  // a LIST of shipping methods
    category: Category,         // a string in [Audio, Video] (ENUM)
    price: union(Num, Price),   // price expressed in dollars OR in another currency (UNION)
    dim: tuple([Num, Num])      // width x height (TUPLE)
});

var Url = subtype(Str, function (s) {
    return s.indexOf('http://') === 0;
});

var Shipping = struct({
    description: Str,
    cost: Num
});

var Category = enums({
    Audio: 0,
    Video: 1
});

var Price = struct({
    currency: Str,
    amount: Num
});

// get an immutable instance
var ipod = new Product({
    name: 'iPod',
    description: 'Engineered for maximum funness.',
    homepage: 'http://www.apple.com/ipod/',
    shippings: [{
        description: 'Shipped to your door, free.',
        cost: 0
    }],
    category: 'Audio',
    price: {currency: 'EUR', amount: 100},
    dim: [2.4, 4.1]
});

Setup

Node

npm install tcomb

var t = require('tcomb');

Browser

This library uses a few ES5 methods

• Array#forEach()
• Array#map()
• Array#some()
• Array#every()
• Object#keys()

you can use es5-shim to support old browsers

<!--[if lt IE 9]>
<script src="es5-shim.min.js"></script>
<![endif]-->
<script type="text/javascript" src="tcomb.js"></script>
<script type="text/javascript">
    console.log(t);
</script>

Tests

Run

mocha

in the project root.

What's a type?

A type is a function T such that

1. T has signature T(values, [mut]) where values depends on the nature of T and the optional boolean arg mut makes the instance mutable (default false)
2. T is idempotent: new T(new T(values)) "equals" new T(values)
3. T owns a static function T.is(x) returning true if x is a instance of T

Note: 2. implies that T can be used as a default JSON decoder

Api

primitive(name, is)

Used internally to define JavaScript native types:

• Nil: null and undefined
• Str: strings
• Num: numbers
• Bool: booleans
• Arr: arrays
• Obj: plain objects
• Func: functions
• Err: errors

Example

Str.is('a'); // => true
Nil.is(undefined); // => true
Nil.is(0); // => false

struct(props, [name])

Defines a struct like type.

• props hash field name -> type
• name optional string useful for debugging

Example

// define a struct with two numerical props
var Point = struct({
    x: Num,
    y: Num
});

Methods are defined as usual

Point.prototype.toString = function () {
    return '(' + this.x + ', ' + this.y + ')';
};

Building an instance is simple as

"use strict";
var p = new Point({x: 1, y: 2});
p.x = 2; // => TypeError, p is immutable

p = new Point({x: 1, y: 2}, true); // now p is mutable
p.x = 2; // ok

Some meta informations are stored in a meta hash

Point.meta = {
    kind: 'struct',
    props: props,
    name: name
};

is(x)

Returns true if x is an instance of Point.

Point.is(p); // => true

update(instance, updates, [mut])

Returns an instance with changed props, without modifying the original.

Point.update(p, {x: 3}); // => new Point({x: 3, y: 2})

union(types, [name])

Defines a types union.

• types array of types
• name optional string useful for debugging

Example

var Circle = struct({
    center: Point,
    radius: Num
});

var Rectangle = struct({
    bl: Point, // bottom left vertex
    tr: Point  // top right vertex
});

var Shape = union([
    Circle, 
    Rectangle
]);

// in order to use Shape as a constructor the function `dispatch(values) -> Type` 
// must be implemented
Shape.dispatch = function (values) {
    assert(Obj.is(values));
    return values.hasOwnProperty('center') ?
        Circle :
        Rectangle;   
};

var shape = new Shape({center: {x: 1, y: 2}, radius: 10});

Some meta informations are stored in a meta hash

Shape.meta = {
    kind: 'union',
    types: types,
    name: name
};

is(x)

Returns true if x belongs to the union.

Shape.is(new Circle({center: p, radius: 10})); // => true

maybe(type, [name])

Same as union([Nil, type]).

var MaybeStr = maybe(Str);

MaybeStr.is('a');     // => true
MaybeStr.is(null);    // => true
MaybeStr.is(1);       // => false

Some meta informations are stored in a meta hash

MaybeStr.meta = {
    kind: 'maybe',
    type: type,
    name: name
};

enums(map, [name])

Defines an enum of strings.

var Direction = enums({
    North: 0, 
    East: 1,
    South: 2, 
    West: 3
});

Some meta informations are stored in a meta hash

Direction.meta = {
    kind: 'enums',
    map: map,
    name: name
};

is(x)

Returns true if x belongs to the enum.

Direction.is('North'); // => true

tuple(types, [name])

Defines a tuple whose coordinates have the specified types.

var Args = tuple([Num, Num]);

var a = new Args([1, 2]);

Some meta informations are stored in a meta hash

Args.meta = {
    kind: 'tuple',
    types: types,
    name: name
};

is(x)

Returns true if x belongs to the tuple.

Args.is([1, 2]);      // => true
Args.is([1, 'a']);    // => false, second element is not a Num
Args.is([1, 2, 3]);   // => false, too many elements

update(instance, index, element, [mut])

Returns an instance without modifying the original.

Args.update(a, 0, 2);    // => [2, 2]

subtype(type, predicate, [name])

Defines a subtype of an existing type.

var Int = subtype(Num, function (n) {
    return n === parseInt(n, 10);
});

// points of the first quadrant
var Q1Point = subtype(Point, function (p) {
    return p.x >= 0 && p.y >= 0;
});

// constructor usage
var p = new Q1Point({x: -1, y: -2}); // => fail!

Some meta informations are stored in a meta hash

Int.meta = {
    kind: 'subtype',
    type: type,
    predicate: predicate,
    name: name
};

is(x)

Returns true if x belongs to the subtype.

Int.is(2);      // => true
Int.is(1.1);    // => false

list(type, [name])

Defines an array where all elements are of type type.

var Path = list(Point);

// costructor usage
var path = new Path([
    {x: 0, y: 0}, 
    {x: 1, y: 1}
]);

Some meta informations are stored in a meta hash

Path.meta = {
    kind: 'list',
    type: type,
    name: name
};

is(x)

Returns true if x belongs to the list.

Path.is([{x: 0, y: 0}, {x: 1, y: 1}]);      // => true

func(Arguments, f, [Return], [name])

Defines a function where the arguments and the return value are checked.

var sum = func(tuple([Num, Num]), function (a, b) {
    return a + b;
}, Num);

sum(1, 2); // => 3
sum(1, 'a'); // => fail!

Useful methods

Return an instance without modifying the original.

Path.append(path, element, [mut]);
Path.prepend(path, element, [mut]);
Path.update(path, index, element, [mut]);
Path.remove(path, index, [mut]);
Path.move(path, from, to, [mut]);

Utils

fail(message)

assert(guard, [message])

// make properties immutables unless `unless` is `true`
freeze(obj_or_arr, [unless])

// adds the properties of `y` to `x`. If `overwrite` is falsy
// and a property already exists, throws an error
mixin(x, y, [overwrite])

// array manipulation
append(arr, element);
prepend(arr, element);
update(arr, index, element);
remove(arr, index);
move(arr, from, to);

More examples

How to extend a struct

var Point3D = struct(mixin(Point.meta.props, {
    z: Num
}));

var p = new Point3D({x: 1, y: 2, z: 3});

Deep update of a struct

var c = new Circle({center: {x: 1, y: 2}, radius: 10});

// translate x by 1
var c2 = Circle.update(c, {
    center: Point.update(c.center, {
        x: c.center.x + 1
    })
});

Write a general JSON Decoder

// (json, T, mut) -> instance of T
function decode(json, T, mut) {
    if (T.fromJSON) {
        return T.fromJSON(json, mut);
    }
    switch (T.meta.kind) {
        case 'struct' :
            var values = {};
            var props = T.meta.props;
            for (var prop in props) {
                if (props.hasOwnProperty(prop)) {
                    values[prop] = decode(json[prop], props[prop], mut);    
                }
            }
            return new T(values, mut);
        case 'union' :
            assert(Func.is(T.dispatch));
            return decode(json, T.dispatch(json), mut);
        case 'maybe' :
            return Nil.is(json) ? undefined : decode(json, T.meta.type, mut);
        case 'tuple' :
            return freeze(json.map(function (x, i) {
                return decode(x, T.meta.types[i], mut);
            }, mut));
        case 'subtype' :
            var x = decode(json, T.meta.type, mut); 
            assert(T.meta.predicate(x));
            return x;
        case 'list' :
            return freeze(json.map(function (x) {
                return decode(x, T.meta.type, mut);
            }), mut);
        default :
            return json;
    }
};

Copyright & License

Copyright (C) 2014 Giulio Canti - Released under the MIT License.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.