Schema

Composable type predicates for runtime type checking.

Getting Started

Import the schema namespace. Using $ is recommended for brevity.

import * as $ from './schema.js';
// Or, as an NPM package.
import * as $ from '@minstack/schema';

Construct a new custom schema from pre-existing schemas.

const isPerson = $.object({
  name: $.string(),
  age: $.number(),
});

Infer the type from the custom schema (if required).

type Person = $.SchemaType<typeof isPerson>;

Use the custom schema to narrow the type of a variable.

if (isPerson(value)) {
  // Value type is narrowed to: Person
}

Included Schemas

Simple schemas match the basic TS types.

• string()
• number()
• bigint()
• boolean()
• symbol()
• callable() - Match functions or constructors.
• notDefined()
• defined
• nul()
• notNul()
• nil() - Match null or undefined.
• notNil()
• any() - Match anything as type any.
• unknown() - Match anything as type unknown.

Configurable schemas accept values for matching.

• enumeration(enumType: EnumLike)
• literal(...primitives: Primitive[])
• instance(...constructors: AnyConstructor[])

Composition schemas merge other schemas (or predicates) into more complex schemas.

• union(...predicates: AnyPredicate[])
• intersection(...predicates: AnyPredicate[])
• object(shape: Record<string, AnyPredicate>)
• tuple(...shape: AnyPredicate[])
• record(type?: AnyPredicate)
• array(type?: AnyPredicate)

Utilities which are less commonly used, or normally only used internally.

• schema<T>(predicate: (value: unknown) => value is T)
  • Create a custom schema.
• predicate<T>(predicate: (value: unknown) => value is T)
  • Create a predicate (copy).
• lazy(resolve: () => AnyPredicate)
  • Delay resolving a predicate until needed (for recursive types).
• assert(predicate: AnyPredicate, value: unknown, error?: ErrorLike)
  • Throw if the predicate does not match the value.

Custom Schema

Use the schema utility to create custom schemas with arbitrary validation logic. Creating a factory function which returns a new schema is recommended.

const isNumericString = () => {
  return $.schema<`${number}`>((value) => {
    return (
      typeof value === 'string' &&
      value.trim() !== '' &&
      !Number.isNaN(value as unknown as number);
  });
};

Use the custom schema like any other schema.

const isNumeric = $.union($.number(), isNumericString());

if (isNumeric(value)) {
  // Value type is narrowed to: number | `${number}`
}

Extension Methods

All schemas have basic extension methods.

• .or(predicate: AnyPredicate)
  • Union.
• .and(predicate: AnyPredicate)
  • Intersection.
• .optional()
  • Union with undefined.

An optional string schema could be created as follows.

const isOptionalString = $.string().optional();

All collection schemas (object, tuple, record, array) have additional extension methods.

• .partial()
  • Make all entries optional.
• .required()
  • Make all entries required.

The array schema has an additional extension method.

• .nonEmpty()
  • Match arrays with length > 0.

The object schema has an additional extension method.

• .extend(shape: Record<string, AnyPredicate>)
  • Add new properties or additional constraints (intersections) on existing properties.

Type Assertions

It can be useful to throw an error when a predicate does not match a value.

$.assert($.string(), value, 'value is not a string');

After the assert, the type of value will be narrowed to the predicate type.

The above assertion is equivalent to the following conditional throw.

if (!$.string()(value)) {
  throw new TypeError('value is not a string');
}

Recursive Types

Recursive (self referential) types are possible, but require a few extra steps because type inference doesn't handle self references (Errors: TS7022, TS2454).

First, define the non-recursive part of the schema.

const isNode = $.object({ name: $.string() });

Then, derive the recursive type. This type is needed to explicitly type the recursive schema.

type Node = $.SchemaType<typeof isNode>;
type Tree = Node & { children: Tree[] };

And finally, extend the non-recursive schema to add recursion, and assign the final schema to an explicitly typed variable.

const isTree: $.ObjectSchema<Tree> = isNode.extend({
  children: $.array($.lazy(() => isTree)),
});

Wrong ways to make recursive types

A TS2454 error is raised without the $.lazy wrapper around the self reference. This is because the schema is used before it is defined.

const isTree: $.ObjectSchema<Tree> = isNode.extend({
  // Error: Variable 'isTree' is used before being assigned. ts(2454)
  children: $.array(isTree),
});

A TS7022 error is raised when defining the recursive type in a single step. This is because typescript cannot automatically infer a type which references itself.

// Error: 'isTree' implicitly has type 'any' because it does not have a
//        type annotation and is referenced directly or indirectly in its
//        own initializer. ts(7022)
const isTree = $.object({
  name: $.string(),
  children: $.array($.lazy(() => isTree)),
});