npm i @spaceteams/zap


validation-first schema library with a functional api

by spaceteams

0.3.0 (see all)License:BSD-3-ClauseTypeScript:Built-In
npm i @spaceteams/zap

zap ⚡

Developed during Spacetime at Spaceteams

GitHub Workflow Status NPM Version License

zap is a validation-first schema library with a functional Api.

Some major features are

  • Flexible refinement and validation API
  • Transformation, Coercion and type narrowing
  • JSONSchema support

Quick Start


npm install @spaceteams/zap
yarn add @spaceteams/zap

then import functions directly like this

import { number, object } from "@spaceteams/zap";
const mySchema = object({ a: number() });

Schema Definitions and Type Inference

To get started you only need to know a few schema types and some utility functions. Have a look at this user schema

const user = object({
  name: string(),
  dateOfBirth: optional(date()),
type User = InferType<typeof user>;

This defines a schema user that has a required name and an optional date of birth.

We also infer the type of the Schema which is equivalent to

type User = {
  name: string;
  dateOfBirth?: Date | undefined;

Typeguards and Validations

This schema can now be used as a type guard

function processUser(value: unknown): string {
  if (user.accepts(value)) {
    // typescript infers value to be of type User
    return value.name;
  // in this branch value is of unknown type!
  return "not a user!";

Or if you need full validation errors

const validationErrors = user.validate({ name: 12 });

will print out

{ name: 'value was of type number expected string' }

For the sake of demonstration we use translate() to make the validation object more readable. Actually, name is not a string but an object containing more information about the validation error such as an issue code, the actual value validated against, etc...

Parsing and Coercion

The schema type also comes with a parse function. This function builds a new object that conforms to the schema. By default however, the schema won't be able to convert types. For example

user.parse({ name: "Joe", dateOfBirth: "1999-01-01" });

will throw a validation error because "1999-01-01" is a string and not a Date object. You can fix this problem with coercion like this

const coercedDate = coerce(date(), (v) => {
  if (typeof v === "string" || typeof v === "number") {
    return new Date(v);
  return v;
const user = object({
  name: string(),
  dateOfBirth: optional(coercedDate),

The coerce function applies the Date only if the value is a string or a number and does not touch any other value (including dates).

The usecase of coercion from string to Date is so common that we have coercedDate() as a builtin function

Table of Contents

🚧 A lot of chapters of the documention are still missing. But each chapter links to relevant source code and specs.



spec and source

At the core of zap is the schema interface. All schema functions (like object(), number(), string()...) return an object that implements it. It is defined as

export interface Schema<I, O = I, M = { type: string }> {
  accepts: (v: unknown, options?: Partial<ValidationOptions>) => v is I;
  validate: (
    v: unknown,
    options?: Partial<ValidationOptions>
  ) => ValidationResult<I>;
  validateAsync: (
    v: unknown,
    options?: Partial<ValidationOptions>
  ) => Promise<ValidationResult<I>>;
  parse: (v: unknown, options?: Partial<Options>) => ParseResult<I, O>;
  parseAsync: (
    v: unknown,
    options?: Partial<Options>
  ) => Promise<ParseResult<I, O>>;
  meta: () => M;

which is quite a handful.

Let us start with accepts and validate. Both get a value of unknown type and run validations on it. While validate builds a complete ValidationResult containing all found validation errors, accepts only returns a typeguard and is slightly more efficient thatn validate. The type of this validation is the first generic type I. If you don't care for the other two generic types you can write such a schema as Schema<I>. Both functions also accept a set of options. Tey currently include earlyExit (default false) which will stop validation on the first issue and withCoercion (default false) which will also coerce values on Validation (see coerce)

There is an async version of both validate and parse available. These are needed if you use async refinements.

Validation is great if you want to check if a value conforms to a schema, but sometimes you want to coerce, tranform a value or strip an object of additional fields. For these cases you want to call parse(). This function returns a Result of type

type ParseResult<I, O> = {
  parsedValue?: O;
  validation?: ValidationResult<I>;

parsedValue is defined if parsing was successful, otherwise validation contains the validation issues found. Note that parse has two generic types: I and O. The first is the type the Schema accepts. The second one is O the output type. By default it is equal to I but can be changed with transform() or narrow() (see transform and narrow). Like validate it accepts options, so you can configure the validation step and also ParsingOptions that control the parsing behaviour. There is strip (default true) that will remove all additional properties from objects and skipValidation (default false) if you do not want to validate, but directly run the parse step.

The last method defined is meta(). It returns an object that describes the schema. For example items(array(number()), 10).meta() will return an object of type

  type: "array";
  schema: Schema<number, number, { type: "number"; }>;
} & {
  minItems: number;
  maxItems: number;

You can use this object to traverse the schema tree (via the schema attribute, that is present because array contains another schema) or reflect on validation rules (for example minItems is set to 10 in the example). This object is used heavily in utility functions like toJsonSchema() or partial().

To make traversing the meta object tree easier we have Optics


spec and source

How we represent validation results is quite unique to zap. Let's say you have a schema.

const schema = object({
  a: object({
    b: array(number()),
  c: string(),

then a validation might look something like this

const validation = {
  a: {
    b: [undefined, new ValidationIssue(...)]
  c: new ValidationIssue(...)

Our validation result resembles the schema closely which can be quite convenient to work with. If you want a more traditional representation however, we got you covered. The path validation functions convert between this representation and a list of path-issue pairs.

As you have seen before you can also translate this validation. By default translate will use a canonical way to transform each validation error into a string but you can customize that behavior.

Let us have a closer look at the ValidationResult since its type is a bit off-putting at first (and the type of Validation is even more complicated)

export type ValidationResult<T, E = ValidationIssue> =
  | Validation<T, E>
  | undefined;

So it is dependent on T and an error type E. By default the error type is a ValidationIssue. This class extends Error so it can be thrown nicely and it contains a ValidationIssueCode, a custom error message, the validated value and a list of args to give further information about the validation. Using the translate method you can transform a ValidationResult<T, ValidationIssue> into a ValidationResult<T, string> containing user readable validation errors. The default translator is a bit technical though, so it is up to you to translate ValidationIssues for your users.

The ValidationResult can now either by undefined indicating a success or a Validation<T, E> indicating a failure. You can check this with isSuccess and isFailure functions. The ValidationResult has a pretty complicated typedefinition but it tries to resamble a deeply partial T with ValidationIssues instead of the actual types. Consider this validation:

type Value = {
  array: number[];
  nested: {
    id: string;
const validation: Validation<Value, string> = {
  array: [undefined, "validation error"],
  nested: "object invalid",

This is a validation of type Validation<Value, string> so it validates Value and uses a string to describe validation issues. In the example the second entry of array has a validation error and the nested object itself has a validation error.

By default zap will keep on validation even if an issue has been encountered (you can change this with the earlyExit flag). We even keep on validating through an and schema (aka Intersection type) and merge the individual Validation objects. This is especially helpful when validating complex forms.


spec and source

Out of the box zap supports a lot of validation methods. Methods like length for strings or after for dates. These validation methods (or refinements) are described together with their applicable schemas.

You can build custom validation methods, however. And the simplest way is the validIf function

validIf(number(), (v) => v % 2 === 0, "must be even");

This function creates a validation error if the given number is not even.

The next powerful refinement function is just called refine. It takes a schema and a function (v: I, ctx: RefineContext<P>) => void | ValidationResult<P>. Where the ctx object contains both the ValidationOptions and helper methods add and validIf. The generic Parameter P is defined as P extends I = I, which means that it is I by default or it narrows it further.

Refine supports three styles of refinement:

const schema = object({ a: string(), b: number() });
const defaultStyle = refine(schema, ({ a, b }) => {
  if (a.length !== b) {
    return {
      a: new ValidationIssue("generic", "a must have length of b", v),
const builderStyle = refine(schema, ({ a, b }, { add }) => {
  if (a.length !== b) {
      a: new ValidationIssue("generic", "a must have length of b", v),
const inlineStyle = refine(schema, ({ a, b }, { validIf }) => ({
  a: validIf(a.length === b, "a must have length of b"),

Here we refine an object {a: string, b: number} so that the string a has length b. In the first style the ValidationResult itself is returned. This is very similar to the refine method the Schema supports. The second style is using the add method. This approach is useful if you want to iteratively collect validation errors and have them merged into a final validation result. And finally, there is an inline style using the validIf method. The advantage of refine over the simpler validIf is that you can add validation errors anywhere in the ValidationResult. For exmple you could validate the age field and write the error inside the name field. Also you can do narrowing:

  object({ id: optional(string()) }),
  (v, ctx: RefineContext<{ id: string }>) => ({
    id: ctx.validIf(v !== undefined, "must be present"),

which will result in a type {id: string} and not {id?: string | undefined}.

Most of zap's built-in validation functions are implemented using refineWithMetainformation. They add meta-information that can be picked up and interpreted by utility functions like toJsonSchema

There are also validIfAsync, refineAsync and refineAsyncWithMetaInformation. Consider validation of a user registration

// call the backend
const userAvailable = (_username: string) => Promise.resolve(true);

export const userRegistration = object({
  username: validIfAsync(
    "this username is already taken"

This will call the function userAvailable if userName is a string and await the result. You should of course consider to debounce, deduplicate and cache your requests to the backend depending on your usecase. To use this schema you have to call validateAsync and refineAsync, the synchronous versions will result in validation errors.


spec and source

By default, a schema will not try to convert values during the parse step. In that case, the parse function will return its inputs without changing them. If you want to parse values like "1998-10-05" as dates however, you will need coercion.

coerce takes a schema and a function (v: unknown) => unknown that may or may not convert the given value. Currently, this function is applied during parse before the validation step and again for the actual parsing. Coercion is not applied in accepts or validate so a coercedDate() will still accept only dates (it is a Schema<Date> after all!). You can override this behaviour using the withCoercion option.

The predefined coerced schemas are coercedBoolean, coercedDate , coercedNumber, coercedString and json

All except json are implemented using the default coercion of javascript. Note that this comes with all the pitfalls and weirdnesses of javascript. For example [] is coerced to 0, '' or true with to coercedNumber, coercedString and coercedBoolean respectively.

json is a wrapper for JSON.parse and will attempt to coerce a string into an object. This is useful when parsing request bodies like this


Transform and Narrow

spec and source

After you parsed a value, you might want to further transform it. For example the schema defaultValue(optional(number()), 42) will parse undefined to 42. This schema has type Schema<number | undefined, number> indicating that it will still accept undefined but will always parse to a number.

The defaultValue function is implemented using narrow(). This function takes a schema and a projection function (v: O) => P where P extends O. This means that the narrowed type must still be assignable to the ouput type.

If you need even more powerful transformations you can use transform(). This function takes a schema and an arbitrary transformation (v: O) => P. This is very similar to narrow() except for the missing contraint on P. With this function you can implement a schema like this

transform(array(number()), values => Math.max(...values))

This schema accepts an array of numbers and parses them into their maximum value. This schema has a type like Schema<number[], number>.

Simple Schema Types


spec and source

bigInt() accepts BigInt values.

There is a coercedBigInt that uses standard JS coercion using the BigInt constructor.

Most of the Number refinements also work for bigInt.


spec and source

boolean() accepts boolean values. It is equivalent to literals(true, false) but creates slightly more precise validation issues.

There is a coercedBoolean that uses standard JS coercion to boolean.


spec and source

date() validates Date objects and accepts only if they point to an actual time by validating them against isNaN.

There is a coercedDate that uses the Date constructor if the value is string or number.

Validation Functions

before - accept dates before the given value
after - accept dates after the given value


spec and source

nativeEnum validates native typescript enum types (not to be confused with a union of literals).

Defining a schema

enum E {
  b = 12,
  c = "c",
const schema = nativeEnum(E);

results in a type Schema<E> that accepts the enum values E.a through E.c or their actual values 0, 12 and "c".

You can also define a nativeEnum from a constant object

const constEnum = nativeEnum({
  a: "a",
  b: 12,
  c: "c",
} as const);

resulting in a type Schema<"a" | "c" | 12>.


spec and source

The literal method expresses typescript literals. Examples include


null and undefined must be expressed using nullSchema or through the optionals.

There is also literals to create a union of literals like

literals(true, false);
literals(1, 2, "a", "some enum");

If you need even more power you can use or.

Null Schema

spec and source

This schema accepts only null. Also have a look at optionals.


spec and source

The number schema accepts any number that is not NaN.

Note that typeof NaN === "number" in javascript

There is coercion function coercedNumber that uses javascript Number constructor.

Validation Functions

nan - accepts if value is NaN
positive - accepts positive numbers (>0)
nonPositive - accepts non-positive numbers (<=0)
negative - accepts negative numbers (<0)
nonNegative - accepts non-negative number (>=0)
integer - accepts integer numbers
multipleOf - accepts numbers that are multiples of the given value (also works for floating points)
exclusiveMaximum - accepts numbers that are strictly smaller than a threshold (<)
exclusiveMinimum - accepts numbers that are strictly greater than a threshold (>)
maximum - accepts numbers that are smaller than or equal to a threshold (<=)
minimum - accepts numbers that are greater than or equal to a threshold (>=)


spec and source

The string schema accepts any string. There is a coercion function coercedString that uses jacscripts String constructor. Also note that you can use json to coerce a json string into an object (see schema-section)

Validation Functions

minLength - accepts strings with a length greater than or equal to a threshold (>=)
maxLength - accepts strings with a length smaller than or equal to a threshold (<=)
length - accepts strings with a length equal to a threshold
nonEmptyString - accepts strings that have minLength 1
pattern - accepts strings that match a regular expression (using RegExp.test)
startsWith - accepts strings that start with the given string
endsWith - accepts strings that end with the given string

Composite Schema Types


spec and source


spec and source


spec and source

The object schema is perhaps the most common and the most complex schema of the bunch. With object you can describe what corresponds to an interface in typescript. The example from the spec

const schema = object({
  id: number(),
  name: array(string()),
  description: optional(string()),
  nested: object({
    user: string(),

will be infered to

type S = {
  id: number;
  name: string[];
  nested: {
    user: string;
} & {
  description?: string | undefined;

This shows that nesting works and that we do some fixup work for optional types so that they are also partial in the schema. That means this schema accept both { id: 1, name: [], nested: { user: "" }} and { id: 1, name: [], description: undefined, nested: { user: "" }}.

You can access any nested schema using the meta function like this schema.meta().schema.name.meta().schema or more succinct into(get(schema, "name")).

By default this schema will strip additional properties on parse. You can avoid this by calling schema.parse(value, { strip: false }).


An object schema allows for additional property to be present during validation and will strip them during parsing. If you wrap it with the schema with the strict method it will return a validation error containing the first additional property. A strict schema will also fail to parse objects with additional fields.


Sometimes you want to allow additional properties only if they are valid according to some other schema. In that case (instead of trying to build it using logical operators) you can express it like this

const newSchema = catchAll(schema, number());

the infered type is S & Record<string, number> and the catchall schema can be accessed using newSchema.meta().additionalProperties

IsInstance and fromInstance

you can add an instanceOf check using the isInstance method. That way you can ensure that the prototype property is correctly set. You can also create a schema from an instance using fromInstance. Note that this will only validate the prototype and not any of the contents of your type.

The date method is implemented as a refinement on fromInstance(Date)


The logical operator and can be used to extend an object

and(schema, object({ more: string() }));

While this results in a schema that adds the more property to the schema it also yields an and type schema that does not compose well with other methods like omit and pick. This is because the and operator is more general and works for arbitrary types. That is why you should prefer the merge method. Merge behaves like and except that it can only be called with object schemas and will result in an object schema. So you can do

const m = merge(schema, object({ more: string() }));

and also apply omit, pick, keys, strict etc to it.

Omit and Pick

Omit and pick both correspond to the respective typescript types. You can do

omit(schema, "id", "description");
pick(schema, "nested");

to get Omit<S, 'id' | 'description'> and Pick<S, 'nested'> as the result type.


The keys method returns a literals schema of the keys of the schema. You can get the keys as a list using keys(schema).meta().literals which is typed in our example as ("id" | "name" | "description" | "nested")[]


spec and source


spec and source


spec and source


spec and source


spec and source



spec and source

The and method (aka intersection) is equivalent to the & operator. Say you want to describe a type { id: string } & { name: string } then you would write this as

const entityWithName = and(
  object({ id: string() }),
  object({ name: string() })

Such a schema accepts only if all subschemas accept and returns the sum of all validation errors (using the mergeValidations method on all subschemas).

This method is (like the &) defined on arbitrary types. However, you most likely need it to combine interfaces. In those cases you are better of using merge.

Discrimated Union

spec and source

The discriminated union is similar to the more general or operator. While or can be applied to any type, the discrimated union only works for objects. Say you have CRUD operations like this

const commands = [
    type: literal("create-recipe"),
    recipe: omit(RecipeSchema, "id"),
    type: literal("delete-recipe"),
    id: get(RecipeSchema, "id"),
const CreateOrDelete = or(...commands);

This creates a schema CreateOrDelete schema that will accept either a create or a delete command. Internally, the schema will try to validate against each subschema one after the other. A similar process is done for parsing.

In the commands in our example the sole purpose of the type attribute is to differentiate the commands from one another. This can be expressed in this way

const CreateOrDelete = discriminatedUnion("type", ...commands);

which will result in a schema that selects the sub-schema based on the value of the type attribute. The matching schema will be used for validation and parsing. This can be slightly more efficient and will also result in clearer validation errors.


spec and source

With the not operator you can describe a schema like this

const schema = and(
  not(object({ a: integer(number()) })),
  object({ a: number() })

The not operator is a Schema<unknown> because is does not give any information about the type of the current value. It accepts if the inner schema rejects.

However, we can write a clearer schema using a custom validation

const nonNatural = validIf(
  (v) => !isInteger(v),
  "must not be an natural number"
const schema = object({ a: nonNatural });


spec and source

The or operator (aka union) is the equivalent to the | operator. Say you want to describe a type number | { name: string } then you would write this as

const numberOrBoxedString = or(number(), object({ name: string() }));

Such a schema accepts if at least one subschema accepts. It returns the validation error of the last sub schema. For parsing it finds the first accepting schema and parses the value into this one.

If you just want to create a union of literals you can use the literals method.


spec and source

The xor operator is a stricter or that accepts if and only if one subschema accepts. It uses that schema for validation and parsing. If more than one schema accepts it returns an xor validation error otherwise it returns the validation error of the last failing sub schema.



spec and source

If you need to write recursive types you can do this with the lazy method

interface Category {
  subCategories?: Category[] | undefined;
const schema: Schema<Category, Category, { type: "object" }> = lazy(() =>
    subCategories: optional(array(schema)),

Note that you need help with the type inference here. Circular types are not supported.


spec and source

If you want to access a schema in a nested structure like this

const schema = object({
  a: object({
    b: object({
      c: number(),
  moreFields: number(),

you can use the meta object:


this is feels cumbersome and a bit hard to read so we built a function get

get(get(get(schema, "a"), "b"), "c");

With this function you can also reach into Maps, Records, Tuples and Procedures. For composite schemas that only have one nested schema like Array, Set and Promise we have into. For example


will return the string() schema.

These functions are inspired by lenses and functional references but are unfortunately not as powerful. They are directly applied onto a schema, you are only able to reach down one level and you cannot mutate schemas. So not at all optics, but it is the spirit that counts!

If you want to mutate an existing schema you could do that together with merge and omit

merge(omit(schema, "a"), get(get(schema, "a"), "b"));

this replaces the field a by c resulting in a Schema<{ c: number; moreFields: number(); }>

Optional, Required, Nullable & Nullish

Optional: spec and source

To express

Partial & DeepPartial

Partial: spec and source DeepPartial: spec and source


spec and source

The default validation result is an object that resembles the structure of the schema. While this is quite handy and easy to use in Javascript and Typescript, such a structure is very hard to generate and work with in languages that do not support structural typing (like most running on the JVM).
This is why we support PathValidationResult. A structure like this:

const validation = {
  a: {
    b: [undefined, new ValidationIssue(...)]
  c: new ValidationIssue(...)

would be represented as

const pathValidation = [
  { path: ".a.b", issue: new ValidationIssue(...) },
  { path: ".c", issue: new ValidationIssue(...) }

you can transform between the two representations with toPathValidation and fromPathValidation.

Special care is taken to also transform Set and Map. This is mediated using a list of PathValidationHints.


spec and source

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