narrows

Super lean and simple object validation with TypeScript support.

narrows is…

• composable: use any validator inside another
• extensible: validators are simple functions — easily write your own
• safe: in TypeScript, validators turn your variables into static types
• tiny: no dependencies, fewer than 50 lines of code and barely 500b gzipped

Examples

Easily validate complex types by using simple validators together:

import { number, record, string } from "narrows";

const validate = record({
  a: string,
  b: number
});

validate({ a: "abc", b: 123 }); // true
validate(42); // false

If you're using TypeScript, narrows is even more useful: it uses type guards to pull static type information from your validators.

import { number, record, string } from "narrows";

const validate = record({
  a: string,
  b: number
});

const foo: unknown = {
  a: "abc",
  b: 123
};

if (validate(foo)) {
  foo; // type narrowed to { a: string; b: number; }
}

Extending

It's super easy to write your own validators! They're just functions that return true or false. Check it out:

import { all, number } from "narrows";

const positive = x => x > 0; // this is a validator!
const validate = all(number, positive);

validate(5); // true
validate(-1); // false

It's also common to write validator creators: functions that return validators. This lets you apply custom behavior when you use them.

import { all, number } from "narrows";

const greaterThan = x => y => y > x;
const validate = all(number, greaterThan(10));

validate(11); // true
validate(5); // false

In TypeScript, your validators should be user-defined type guards: functions in the form of (x: unknown) => x is T. This lets TypeScript extract static type information.

import { record, number } from "narrows";

type Actions = "increment" | "decrement";
const action = (x: unknown): x is Actions =>
  x === "increment" || x === "decrement";

const validate = record({
  type: action,
  amount: number
});

const foo: unknown = {
  type: "increment",
  b: 1
};

if (validate(foo)) {
  foo; // type narrowed to { type: "increment" | "decrement"; amount: number; }
}

API

Primitive Validators

Validate that a variable is a boolean, number, string, null/undefined or a literal value.

boolean

Returns true if and only if the argument is a boolean.

import { boolean } from "narrows";

boolean(false); // true
boolean(0); // false

number

Returns true if and only if the argument is a number.

import { number } from "narrows";

number(0); // true
number("string"); // false

string

Returns true if and only if the argument is a string.

import { string } from "narrows";

string("test"); // true
string(true); // false

empty

Returns true if and only if the argument is undefined.

import { empty } from "narrows";

empty(undefined); // true
empty("string"); // false

nil

Returns true if and only if the argument is null.

import { nil } from "narrows";

nil(null); // true
nil("string"); // false

literal

Returns true if and only if the argument is strictly equal to the given value. Two values are strictly equal if they are primitives with the same value, or references to the same object.

import { literal } from "narrows";

const validate = literal(5);

validate(5); // true
validate(6); // false

In TypeScript, this will narrow to the widened type instead of the constant (e.g. number instead of 5). To fix this, you can assert the type manually:

import { literal } from "narrows";

let validate = literal(5 as 5); // TypeScript 3.3 and below
validate = literal(5 as const); // TypeScript 3.4 and above

const foo: unknown = 5;

if (validate(foo)) {
  foo; // type narrowed to 5
}

Complex Validators

Validate that a variable conforms to a certain shape, or is an instance of a constructor.

object

Returns true if and only if each property in an object matches the given validator.

import { object, number } from "narrows";

const validate = object(number);

validate({ a: 1, b: 2 /* ... */ }); // true
validate({ a: "a", b: "b" /* ... */ }); // false

const foo: unknown = {
  a: 1,
  b: 2
  // ...
};

if (validate(foo)) {
  foo; // type narrowed to { [key: string]: number }
}

array

Returns true if and only if each element in an array matches the given validator.

import { array, number } from "narrows";

const validate = array(number);

validate([1, 2 /* ... */]); // true
validate(["a", "b" /* ... */]); // false

const foo: unknown = [1, 2 /* ... */];

if (validate(foo)) {
  foo; // type narrowed to number[]
}

instance

Returns true if and only if a variable matches the given constructor.

import { instance } from "narrows";

const validate = instance(Date);

validate(new Date()); // true
validate({ a: 1 }); // false

const foo: unknown = new Date();

if (validate(foo)) {
  foo; // type narrowed to Date
}

Schema Validators

Validate that all members of a type match corresponding validators in the given schema.

record

Given an object of validators, returns true if and only if each validator in that object matches the corresponding property in the object being tested.

If extra properties are present, this function still returns true — but the extra properties won't be reflected in the narrowed type.

import { number, record, string } from "narrows";

const validate = record({
  a: string,
  b: number
});

const foo: unknown = {
  a: "abc",
  b: 123,
  c: true
};

if (validate(foo)) {
  foo; // type narrowed to { a: string; b: number; }
}

tuple

Given an array of validators, returns true if and only if each validator in that array matches the corresponding element in the array being tested.

If extra elements are present, this function still returns true — but the extra elements won't be reflected in the narrowed type.

import { number, string, tuple } from "narrows";

const validate = tuple([string, number]);

const foo: unknown = ["abc", 123, true];

if (validate(foo)) {
  foo; // type narrowed to[string, number]
}

Combinators

Take in two or more validators and return a single validator that applies all of their rules.

any

Returns true if the argument matches any of the given validators.

Narrows to a union of the validator types.

import { any, number, string } from "narrows";

const validate = any(number, string);

validate(0); // true
validate("one"); // true
validate(false); // false

const foo: unknown = 0;

if (validate(foo)) {
  foo; // type narrowed to number | string
}

all

Returns true if the argument matches all of the given validators.

Narrows to an intersection of the validator types.

import { all, number, record, string } from "narrows";

const validate = all(record({ a: number }), record({ b: string }));

validate({ a: 1, b: "2" }); // true
validate({ a: 1 }); // false
validate({ b: "2" }); // false

const foo: unknown = {
  a: 1,
  b: "2"
};

if (validate(foo)) {
  foo; // type narrowed to { a: number; b: string; }
}

optional

Returns true if the argument matches the given validator or is undefined.

Narrows to a union of the validator type and undefined.

import { optional, number } from "narrows";

const validate = optional(number);

validate(0); // true
validate(undefined); // true
validate("string"); // false

const foo: unknown = undefined;
if (validate(foo)) {
  foo; // type narrowed to number | undefined
}

nullable

Returns true if the argument matches the given validator or is null.

Narrows to a union of the validator type and null.

import { nullable, number } from "narrows";

const validate = nullable(number);

validate(0); // true
validate(null); // true
validate("string"); // false

const foo: unknown = null;
if (validate(foo)) {
  foo; // type narrowed to number | null
}

TypeOf

TypeScript type that extracts the validated type from a validator function.

import { boolean, number, record, string, tuple, TypeOf } from "narrows";

const stringValidator = string;

type Foo = TypeOf<typeof stringValidator>; // string

const complexValidator = record({
  a: string,
  b: tuple(boolean, number)
});

type Bar = TypeOf<typeof complexValidator>; // { a: string; b: [boolean, number] }