This small library provides a simple schema validation system for JavaScript/TypeScript. The library has basic types with opportunities for extending.

Installation

npm install @esmj/schema

Quick Start

Get started with @esmj/schema in seconds:

import { s } from '@esmj/schema';

// Define a schema
const userSchema = s.object({
  name: s.string(),
  age: s.number(),
  email: s.string().optional()
});

// Parse data
const user = userSchema.parse({
  name: 'John Doe',
  age: 30
});

console.log(user);
// { name: 'John Doe', age: 30 }

// Safe parse with error handling
const result = userSchema.safeParse({
  name: 'Jane',
  age: 'invalid'
});

if (result.success) {
  console.log(result.data);
} else {
  console.error(result.error.message);
}

With Extensions:

import { s } from '@esmj/schema/full';

const schema = s.object({
  username: s.string().trim().toLowerCase().min(3).max(20),
  age: s.number().int().positive().min(18),
  tags: s.array(s.string()).min(1).unique()
});

const result = schema.parse({
  username: '  JohnDoe  ',
  age: 25,
  tags: ['developer', 'typescript']
});
// { username: 'johndoe', age: 25, tags: ['developer', 'typescript'] }

Why Use `@esmj/schema`?

@esmj/schema is a lightweight and flexible schema validation library designed for developers who need a simple yet powerful way to validate and transform data. Here are some reasons to choose this package:

TypeScript First: Built with TypeScript in mind, it provides strong type inference—even for deeply nested and complex schemas.
Extensibility: Easily extend the library with custom logic, refinements, and preprocessors using the extend function.
Rich Features: Includes advanced features like preprocessing, transformations, piping, refinements, and robust error collection (abortEarly), which are not always available in similar libraries.
Actionable Error Handling: Collect all validation errors at once for better debugging and user experience, with clear and consistent error structures.
Lightweight: No dependencies and a small footprint make it ideal for projects where performance and simplicity are key.
Customizable: Offers fine-grained control over validation, error handling, and schema composition.
Performance: Optimized for speed, making it one of the fastest schema validation libraries available.
Modular: Import only what you need with separate string, number, and array extension modules to minimize bundle size.

Performance Highlights

Schema Creation: Create schemas at up to 4 370 618 ops/s (0.23 μs latency) with @sinclair/typebox, or 736 810 ops/s (1.36 μs latency) with @esmj/schema. Superstruct and @esmj/schema are also among the fastest for schema creation.
Parsing: Parse data at up to 4 627 714 ops/s (0.22 μs latency) with @zod/mini (note: @zod/mini was observed to consume 200% CPU, while other libraries used only 100% CPU), or 3 142 587 ops/s (0.32 μs latency) with @esmj/schema. ArkType and effect/Schema also show strong parsing throughput.
Error Handling: Efficiently manage errors at up to 2 428 049 ops/s (0.41 μs latency) with @esmj/schema, or 1 386 616 ops/s (0.72 μs latency) with @zod/mini.

These performance metrics make @esmj/schema an excellent choice for both frontend and backend applications where speed and efficiency are critical.

Comparison with Similar Libraries

When choosing a schema validation library, bundle size can be an important factor, especially for frontend applications where minimizing JavaScript size is critical. Here's how @esmj/schema compares to other popular libraries:

Library	Bundle Size (minified + gzipped)
`@esmj/schema`	`~1.6 KB`
Superstruct	~3.2 KB
@sinclair/typebox	~11.7 KB
Yup	~12.2 KB
Zod@3	~13 KB
@zod/mini	~20.5 KB
Joi	~40.4 KB
Zod@4	~40.8 KB
ArkType	~41.8 KB
Effect/Schema	~115.5 KB

Performance Comparison

All benchmarks were measured on Node.js v24.1.0.

Schema Creation Performance

Library	Throughput average (ops/s)	Latency average (μs)
@esmj/schema	736 810.12 ± 3.03%	1.36 ± 3.24%
Zod@3	112 575.50 ± 0.86%	8.88 ± 0.87%
@zod/mini	23 456.07 ± 1.26%	42.64 ± 1.28%
Yup	75 051.06 ± 4.38%	13.36 ± 4.41%
Superstruct	509 401.06 ± 0.80%	1.96 ± 0.80%
Joi	42 455.28 ± 1.27%	23.56 ± 1.30%
`@sinclair/typebox`	`4 370 618.49 ± 1.23%`	`0.23 ± 1.23%`
ArkType	16 282.69 ± 4.14%	61.61 ± 4.38%
effect/Schema	24 919.15 ± 4.31%	40.31 ± 4.78%

Parsing Performance

Library	Throughput average (ops/s)	Latency average (μs)
@esmj/schema	3 142 587.31 ± 0.97%	0.32 ± 0.99%
zod@3	1 018 777.24 ± 0.64%	0.98 ± 0.65%
`@zod/mini`	`4 627 714.90 ± 2.23%`	`0.22 ± 2.36%`
Yup	108 361.49 ± 0.50%	9.23 ± 0.51%
Superstruct	252 904.42 ± 2.20%	3.96 ± 2.44%
Joi	346 094.49 ± 0.65%	2.89 ± 0.65%
@sinclair/typebox	228 711.62 ± 2.03%	4.38 ± 2.23%
ArkType	1 677 066.00 ± 0.58%	0.60 ± 0.59%
effect/Schema	1 060 056.14 ± 0.61%	0.94 ± 0.61%

Error Handling Performance

Library	Throughput average (ops/s)	Latency average (μs)
`@esmj/schema`	`2 428 049.34 ± 0.54%`	`0.41 ± 0.53%`
zod@3	641 504.22 ± 3.67%	1.57 ± 4.38%
@zod/mini	1 386 616.61 ± 0.60%	0.72 ± 0.60%
Yup	98 904.30 ± 0.61%	10.11 ± 0.61%
Superstruct	122 782.09 ± 1.03%	8.15 ± 1.03%
Joi	271 301.11 ± 1.58%	3.69 ± 1.59%
@sinclair/typebox	228 734.49 ± 0.55%	4.37 ± 0.56%
ArkType	258 685.33 ± 1.23%	3.87 ± 1.23%
effect/Schema	165 753.69 ± 0.99%	6.03 ± 1.00%

Note: During the performance tests, @zod/mini was observed to consume 200% CPU, while other libraries used only 100% CPU. This may affect the interpretation of the results, especially in multi-threaded environments.

Usage

Basic Usage

import { s, type Infer} from '@esmj/schema';

const schema = s.object({
  username: s.string().optional().refine((val) => val.length <= 255, {
    message: "Username can't be more than 255 characters",
  }),
  password: s.string().default('unknown'),
  birthday: s.preprocess((value) => new Date(value), s.date()),
  account: s.string().default('0').transform((value) => Number.parseInt(value)).pipe(s.number()),
  money: s.number(),
  address: s.object({
    street: s.string(),
    city: s.string().optional(),
  }).default({ street: 'unknown' }),
  records: s.array(s.object({ name: s.string() })).default([]),
});

type schemaType = Infer<typeof schema>;

const result = schema.parse({
  username: 'john_doe',
  birthday: '2000-01-01T23:59:59.000Z',
  address: { city: 'New York' },
  money: 100,
});

console.log(result);
// {
//   username: 'john_doe',
//   password: 'unknown',
//   birthday: Date('2000-01-01T23:59:59.000Z'),
//   account: 0,
//   money: 100,
//   address: {
//     street: 'unknown',
//     city: 'New York',
//   },
//   records: [],
// }

Modular Extensions

@esmj/schema provides modular extensions that can be imported individually or all together, allowing you to include only the validation helpers you need.

Import Options

// Minimal version (core only, ~1.5 KB)
import { s } from '@esmj/schema';

// Full version (all extensions included, ~4 KB)
import { s } from '@esmj/schema/full';

// String extensions only
import { s } from '@esmj/schema/string';

// Number extensions only
import { s } from '@esmj/schema/number';

// Array extensions only
import { s } from '@esmj/schema/array';

// Mix and match (side-effect imports)
import '@esmj/schema/string';
import '@esmj/schema/number';
import { s } from '@esmj/schema';

Bundle Size Impact

Core only (@esmj/schema): ~1.5 KB gzipped
String extensions (@esmj/schema/string): +~0.8 KB
Number extensions (@esmj/schema/number): +~0.6 KB
Array extensions (@esmj/schema/array): +~0.5 KB
Full (@esmj/schema/full): ~4 KB gzipped (all extensions)

Recommendation: Import only the extensions you need to minimize bundle size.

String Extensions (`@esmj/schema/string`)

String extensions provide common validation and transformation methods for string schemas.

import { s } from '@esmj/schema/string';

const userSchema = s.object({
  username: s.string()
    .trim()              // Remove whitespace
    .toLowerCase()        // Convert to lowercase
    .min(3)              // Minimum 3 characters
    .max(20)             // Maximum 20 characters
    .startsWith('user_'), // Must start with 'user_'
  
  email: s.string()
    .trim()
    .toLowerCase()
    .includes('@')        // Must contain '@'
});

userSchema.parse({
  username: '  USER_John  ',
  email: '  John@Example.com  '
});
// ✓ { username: 'user_john', email: 'john@example.com' }

Available String Methods:

Length validations: min(length), max(length), length(exact), nonEmpty()
Pattern validations: startsWith(prefix), endsWith(suffix), includes(substring)
Transformations: trim(), toLowerCase(), toUpperCase(), padStart(length, char), padEnd(length, char), replace(search, replace)

Number Extensions (`@esmj/schema/number`)

Number extensions provide validation methods for number schemas including range checks and type validations.

import { s } from '@esmj/schema/number';

const productSchema = s.object({
  price: s.number()
    .positive()           // Must be positive
    .min(0.01)           // Minimum value
    .max(999999.99),     // Maximum value
  
  quantity: s.number()
    .int()               // Must be integer
    .positive()
    .min(1)
    .max(1000),
  
  discount: s.number()
    .min(0)
    .max(100)
    .multipleOf(5)       // Must be multiple of 5
});

productSchema.parse({
  price: 29.99,
  quantity: 5,
  discount: 10
});
// ✓ { price: 29.99, quantity: 5, discount: 10 }

Available Number Methods:

Range validations: min(value), max(value), positive(), negative()
Type validations: int(), float(), multipleOf(value), finite()

Array Extensions (`@esmj/schema/array`)

Array extensions provide validation and transformation methods for array schemas.

import { s } from '@esmj/schema/array';

const tagsSchema = s.object({
  tags: s.array(s.string())
    .min(1)              // At least 1 item
    .max(5)              // At most 5 items
    .unique()            // All items must be unique
});

tagsSchema.parse({
  tags: ['javascript', 'typescript', 'node']
});
// ✓ { tags: ['javascript', 'typescript', 'node'] }

Available Array Methods:

Size validations: min(length), max(length), length(exact), nonEmpty()
Content validations: unique()
Transformations: sort(), reverse()

Full Extensions (`@esmj/schema/full`)

The full version includes all string, number, and array extensions in a single import.

import { s } from '@esmj/schema/full';

const productSchema = s.object({
  // String extensions
  name: s.string()
    .trim()
    .min(3)
    .max(100),
  
  sku: s.string()
    .toUpperCase()
    .length(8)
    .startsWith('PROD'),
  
  // Number extensions
  price: s.number()
    .positive()
    .min(0.01)
    .max(999999.99),
  
  stock: s.number()
    .int()
    .min(0),
  
  // Array extensions
  categories: s.array(s.string())
    .min(1)
    .max(5)
    .unique(),
  
  dimensions: s.array(s.number().positive())
    .length(3) // [length, width, height]
});

Custom Error Messages:

All extension methods support custom error messages:

const schema = s.object({
  username: s.string().min(3, {
    message: 'Username is too short! Please use at least 3 characters.'
  }),
  age: s.number().positive({
    message: 'Age must be a positive number.'
  }),
  tags: s.array(s.string()).unique({
    message: 'Duplicate tags are not allowed.'
  })
});

API Reference Summary

Core Types

s.string() - String validation
s.number() - Number validation
s.boolean() - Boolean validation
s.date() - Date validation
s.object(def) - Object validation
s.array(def) - Array validation
s.literal(value) - Literal value validation
s.enum(values) - Enum validation
s.union(schemas) - Union validation
s.any() - Any type
s.null() - Null type
s.undefined() - Undefined type
s.unknown() - Unknown type

Modifiers

.optional() - Makes field optional
.nullable() - Makes field nullable
.nullish() - Makes field optional and nullable
.default(value) - Sets default value
.catch(value) - Returns fallback value on any parse failure

Coerce

s.coerce.string() - Coerce any value to string, then validate
s.coerce.number() - Coerce any value to number, then validate (fails for NaN)
s.coerce.boolean() - Coerce any value to boolean, then validate
s.coerce.date() - Coerce any value to Date, then validate (fails for invalid dates)

Cast

Semantic casting that understands common string representations and rejects ambiguous inputs:

s.cast.boolean() - Cast to boolean; understands 'true'/'false', 'yes'/'no', 'on'/'off', '1'/'0' (case-insensitive); rejects null/undefined/unrecognised strings
s.cast.number() - Cast to number; trims whitespace from strings, accepts booleans (true→1, false→0); rejects null/undefined/empty strings
s.cast.string() - Cast to string; accepts strings, finite numbers, and booleans; rejects null/undefined/objects/NaN/Infinity
s.cast.date() - Cast to Date; accepts ISO strings, finite timestamps, and existing Dates; rejects null/undefined/booleans/empty strings
s.cast.json(schema) - Parse a JSON string and validate the result against a schema; non-string inputs pass through directly; malformed JSON returns a proper validation failure

Transformations

.transform(fn) - Transform value
s.preprocess(fn, schema) - Preprocess before validation
.pipe(schema) - Pipe to another schema
.refine(fn, opts) - Custom validation

String Extensions

Available when importing from @esmj/schema/string or @esmj/schema/full:

Length Validations:

.min(n) - Minimum length
.max(n) - Maximum length
.length(n) - Exact length
.nonEmpty() - Non-empty string

Pattern Validations:

.startsWith(prefix) - Must start with prefix
.endsWith(suffix) - Must end with suffix
.includes(substring) - Must contain substring

Transformations:

.trim() - Remove whitespace
.toLowerCase() - Convert to lowercase
.toUpperCase() - Convert to uppercase
.padStart(length, char) - Pad start
.padEnd(length, char) - Pad end
.replace(search, replace) - Replace text

Number Extensions

Available when importing from @esmj/schema/number or @esmj/schema/full:

Range Validations:

.min(n) - Minimum value
.max(n) - Maximum value
.positive() - Must be positive
.negative() - Must be negative

Type Validations:

.int() - Must be integer
.float() - Must be float (non-integer)
.multipleOf(n) - Must be multiple of n
.finite() - Must be finite

Array Extensions

Available when importing from @esmj/schema/array or @esmj/schema/full:

Size Validations:

.min(n) - Minimum length
.max(n) - Maximum length
.length(n) - Exact length
.nonEmpty() - Non-empty array

Content Validations:

.unique() - All items must be unique

Transformations:

.sort() - Sort array
.reverse() - Reverse array

Schema Types

`s.string(options?)`

Creates a string schema. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const stringSchema = s.string({
  message: 'This is a constant error message.',
});

const stringSchemaFunc = s.string({
  message: (value) => `Custom error: "${value}" is not a valid string.`,
});

`s.number(options?)`

Creates a number schema. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const numberSchema = s.number({
  message: 'This is a constant error message.',
});

const numberSchemaFunc = s.number({
  message: (value) => `Custom error: "${value}" is not a valid number.`,
});

`s.boolean(options?)`

Creates a boolean schema. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const booleanSchema = s.boolean({
  message: 'This is a constant error message.',
});

const booleanSchemaFunc = s.boolean({
  message: (value) => `Custom error: "${value}" is not a valid boolean.`,
});

`s.date(options?)`

Creates a date schema. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const dateSchema = s.date({
  message: 'This is a constant error message.',
});

const dateSchemaFunc = s.date({
  message: (value) => `Custom error: "${value}" is not a valid date.`,
});

`s.object(definition, options?)`

Creates an object schema with the given definition. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const objectSchema = s.object(
  {
    key: s.string(),
    value: s.number(),
  },
  {
    message: 'This is a constant error message.',
  },
);

const objectSchemaFunc = s.object(
  {
    key: s.string(),
    value: s.number(),
  },
  {
    message: (value) => `Custom error: "${JSON.stringify(value)}" is not a valid object.`,
  },
);

`s.array(definition, options?)`

Creates an array schema with the given item definition. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const arraySchema = s.array(s.string(), {
  message: 'This is a constant error message.',
});

const arraySchemaFunc = s.array(s.string(), {
  message: (value) => `Custom error: "${JSON.stringify(value)}" is not a valid array.`,
});

`s.enum(values, options?)`

Creates an enum schema that validates against a predefined set of string values. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const enumSchema = s.enum(['admin', 'user', 'guest'], {
  message: 'This is a constant error message.',
});

const enumSchemaFunc = s.enum(['admin', 'user', 'guest'], {
  message: (value) => `Custom error: "${value}" is not a valid enum value.`,
});

`s.literal(value, options?)`

Creates a literal schema that validates against an exact value. The value can be a string, number, or boolean. This is useful for discriminated unions, API response types, and strict value validation. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

// String literal
const adminSchema = s.literal('admin');
adminSchema.parse('admin'); // ✅ 'admin'
adminSchema.parse('user');  // ❌ throws error

// Number literal
const statusCode = s.literal(200);
statusCode.parse(200); // ✅ 200
statusCode.parse(404); // ❌ throws error

// Boolean literal
const enabled = s.literal(true);
enabled.parse(true);  // ✅ true
enabled.parse(false); // ❌ throws error

// Custom error message
const typeSchema = s.literal('success', {
  message: 'Response type must be "success"',
});

// Custom error function
const versionSchema = s.literal(1, {
  message: (value) => `API version must be 1, received ${value}`,
});

// Discriminated unions with literal
const responseSchema = s.union([
  s.object({
    type: s.literal('success'),
    data: s.string(),
  }),
  s.object({
    type: s.literal('error'),
    error: s.string(),
  }),
]);

// Using multiple literals in union (similar to enum but with type inference)
const roleSchema = s.union([
  s.literal('admin'),
  s.literal('user'),
  s.literal('guest'),
]);

Common Use Cases:

Discriminated Unions: Use literal types to distinguish between different object shapes
API Response Types: Validate exact status codes or response types
Configuration Flags: Validate boolean flags or specific string values
Type Guards: Create strict type validation for specific values

`s.union(definitions, options?)`

Creates a schema that validates against multiple schemas (a union of schemas). The value must match at least one of the provided schemas. You can optionally pass options to customize error messages.

message: Can be either a constant string or a function (value) => string.

const schema = s.union([
  s.string(),
  s.number(),
  s.boolean(),
], {
  message: 'This is a constant error message.',
});

const schemaFunc = s.union([
  s.string(),
  s.number(),
  s.boolean(),
], {
  message: (value) => `Custom error: "${value}" does not match any of the union schemas.`,
});

`s.any()`

Creates a schema that accepts any value.

const anySchema = s.any();

`s.preprocess(callback, schema)`

Creates a schema that preprocesses the input value using the provided callback before validating it with the given schema.

const preprocessSchema = s.preprocess((value) => new Date(value), s.date());

`s.coerce`

The coerce namespace applies a native JS constructor to the input before validation. Unlike s.preprocess, you don't need to write the conversion yourself, and coerce methods provide clear, specific error messages when coercion produces an invalid result.

Method	Coercion applied	Fails when
`s.coerce.string(options?)`	`String(v)`	Never — `String()` always succeeds
`s.coerce.number(options?)`	`Number(v)`	Result is `NaN` (e.g. `'bad'`, `undefined`)
`s.coerce.boolean(options?)`	`Boolean(v)`	Never — `Boolean()` always succeeds
`s.coerce.date(options?)`	`new Date(v)`	Result is an invalid Date (e.g. `'garbage'`)

Note: Boolean('false') is true because 'false' is a non-empty string. This matches JavaScript semantics.

s.coerce.number().parse('42');         // 42
s.coerce.number().parse(true);         // 1
s.coerce.number().parse('bad');        // throws: Cannot coerce "NaN" to a valid number.

s.coerce.string().parse(123);          // '123'
s.coerce.string().parse(null);         // 'null'

s.coerce.boolean().parse(0);           // false
s.coerce.boolean().parse('false');     // true — non-empty string!

s.coerce.date().parse('2024-01-01');   // Date object
s.coerce.date().parse('garbage');      // throws: Cannot coerce "Invalid Date" to a valid date.

// All schema methods chain normally after coerce:
s.coerce.number().refine((v) => v > 0, { message: 'Must be positive' }).parse('5'); // 5

// Custom error message:
s.coerce.number({ message: 'Expected a numeric value' }).parse('bad'); // throws: Expected a numeric value

`s.cast`

Programmer-friendly semantic casting. Unlike s.coerce (raw JS constructors), s.cast understands common string representations and rejects ambiguous inputs like null, undefined, and empty strings.

Method	Accepted inputs	Rejects
`s.cast.string(options?)`	strings, finite numbers, booleans	`null`, `undefined`, objects, `NaN`, `Infinity`
`s.cast.number(options?)`	numbers (incl. booleans `true`/`false`→1/0), trimmed numeric strings	`null`, `undefined`, empty strings, non-numeric strings
`s.cast.boolean(options?)`	booleans, `1`/`0`, `'true'/'false'`, `'yes'/'no'`, `'on'/'off'`, `'1'/'0'`	`null`, `undefined`, unrecognised strings, other numbers
`s.cast.date(options?)`	`Date` objects, ISO strings, finite integer timestamps	`null`, `undefined`, booleans, empty strings, invalid date strings
`s.cast.json(schema, options?)`	JSON strings (parsed), any non-string value (pass-through)	malformed JSON strings

Key differences from s.coerce:

Input	`s.coerce.boolean()`	`s.cast.boolean()`
`'false'`	`true` (non-empty string!)	`false`
`'yes'` / `'no'`	`true` / `true`	`true` / `false`
`null`	`false`	throws

Input	`s.coerce.number()`	`s.cast.number()`
`null`	`0`	throws
`''`	`0`	throws

Input	`s.coerce.string()`	`s.cast.string()`
`null`	`'null'`	throws
`undefined`	`'undefined'`	throws

// boolean
s.cast.boolean().parse('false');     // false — unlike coerce!
s.cast.boolean().parse('yes');       // true
s.cast.boolean().parse('on');        // true
s.cast.boolean().parse('OFF');       // false (case-insensitive)
s.cast.boolean().parse(1);           // true
s.cast.boolean().parse(0);           // false
s.cast.boolean().parse('hello');     // throws: Cannot cast "hello" to boolean...
s.cast.boolean().parse(null);        // throws

// number
s.cast.number().parse('42');         // 42
s.cast.number().parse(' 3.14 ');     // 3.14 — trims whitespace
s.cast.number().parse(true);         // 1
s.cast.number().parse(false);        // 0
s.cast.number().parse(null);         // throws: Cannot cast "null" to a number...
s.cast.number().parse('');           // throws

// string
s.cast.string().parse(123);          // '123'
s.cast.string().parse(true);         // 'true'
s.cast.string().parse(false);        // 'false'
s.cast.string().parse(null);         // throws: Cannot cast "null" to string...
s.cast.string().parse(NaN);          // throws

// date
s.cast.date().parse('2024-01-01');   // Date object
s.cast.date().parse(1704067200000);  // Date object
s.cast.date().parse(null);           // throws: Cannot cast "null" to a valid date.
s.cast.date().parse(true);           // throws

// All schema methods chain normally:
s.cast.number().refine((v) => v > 0, { message: 'Must be positive' }).parse('5'); // 5

// Custom error message:
s.cast.boolean({ message: 'Must be a boolean flag' }).parse('maybe'); // throws: Must be a boolean flag

// json
s.cast.json(s.object({ name: s.string() })).parse('{"name":"Alice"}'); // { name: 'Alice' }
s.cast.json(s.array(s.number())).parse('[1,2,3]');                     // [1, 2, 3]
s.cast.json(s.object({ name: s.string() })).parse({ name: 'Alice' }); // { name: 'Alice' } — pass-through
s.cast.json(s.number()).safeParse('not json');                         // { success: false, error: ... }
s.cast.json(s.number(), { message: 'Invalid JSON' }).parse('bad');     // throws: Invalid JSON

Schema Methods

`parse(value, parseOptions?)`

Parses the given value according to the schema.

const result = stringSchema.parse('hello');

`safeParse(value, parseOptions?)`

Safely parses the given value according to the schema, returning a success or error result.

const result = stringSchema.safeParse('hello'); 
// { success: true, data: 'hello' }

const errorResult = stringSchema.safeParse(123); 
// { success: false, error: { message: 'The value "123" must be type of string but is type of "number".' } }

// Collect all errors (not just the first)
const allErrorsResult = stringSchema.safeParse(123, { abortEarly: false });
console.log(allErrorsResult.errors); // Array of all errors

Note: The error returned by safeParse is not a native Error instance. Instead, it is a plain object with the following structure:

type ErrorStructure = {
  message: string;
  cause?: {
    key?: string;
  };
};

This allows for easier serialization and debugging but may require additional handling if you expect a native Error instance.

`optional()`

Makes the schema optional.

const optionalSchema = stringSchema.optional();

`nullable()`

Makes the schema nullable.

const nullableSchema = stringSchema.nullable();

`nullish()`

Makes the schema nullish (nullable and optional).

const nullishSchema = stringSchema.nullish();

`default(defaultValue)`

Sets a default value for the schema.

const defaultSchema = stringSchema.default('default value');

`catch(catchValue)`

Returns a fallback value whenever parsing fails, instead of throwing or returning an error. Unlike default() which only fires when the input is undefined, catch() fires on any validation failure.

The fallback can be a static value or a function that receives a context object { input, error }:

input — the original raw input value
error — the ErrorStructure with the failure message

Note: The fallback value is returned as-is without re-validation. catch() only intercepts failures from schemas and refinements placed before it in the chain.

// Static fallback
const schema = s.string().catch('unknown');
schema.parse(123);       // 'unknown'
schema.parse('hello');   // 'hello'

// Function fallback with context
const schema2 = s.number().catch((ctx) => {
  console.warn(`Invalid input: ${ctx.input} — ${ctx.error.message}`);
  return 0;
});
schema2.parse('bad');    // 0

// Distinction from default()
s.string().catch('fallback').parse(null);    // 'fallback' — catch fires for null
s.string().default('fallback').parse(null);  // throws — default does not fire for null

`transform(callback)`

Transforms the parsed value using the provided callback.

const transformedSchema = s.string().transform((value) => value.toUpperCase());

`pipe(schema)`

Pipes the output of one schema into another schema for further validation or transformation.

const pipedSchema = s.string().pipe(s.number());

`refine(validation, { message })`

Adds a refinement to the schema with a custom validation function and error message.

const refinedSchema = s.string().refine((val) => val.length <= 255, {
  message: "String can't be more than 255 characters",
});

Error Collection with `abortEarly` Option

Both parse and safeParse accept an optional second argument: parseOptions: { abortEarly?: boolean }

abortEarly (default: true): If true, validation stops at the first error (previous behavior). If false, all validation errors are collected and returned in the errors array.

Example:

const schema = s.object({
  name: s.string(),
  age: s.number(),
  email: s.string()
});

// Default behavior (abortEarly: true)
const result1 = schema.safeParse({
  name: 123,
  age: 'not a number',
  email: 42
});
console.log(result1.success); // false
console.log(result1.errors.length); // 1

// Collect all errors (abortEarly: false)
const result2 = schema.safeParse({
  name: 123,
  age: 'not a number',
  email: 42
}, { abortEarly: false });
console.log(result2.success); // false
console.log(result2.errors.length); // 3

Error Result Structure:

error: The first error encountered (for compatibility)
errors: Array of all errors (when abortEarly: false)

Note:
The abortEarly option is propagated through nested schemas, arrays, unions, and refinements.
This means you get all errors from deeply nested structures when using { abortEarly: false }.

Example Output:

{
  "success": false,
  "error": {
    "message": "Error parsing key \"name\": The value \"123\" must be type of string but is type of \"number\".",
    "cause": { "key": "name" }
  },
  "errors": [
    { "message": "Error parsing key \"name\": ...", "cause": { "key": "name" } },
    { "message": "Error parsing key \"age\": ...", "cause": { "key": "age" } },
    { "message": "Error parsing key \"email\": ...", "cause": { "key": "email" } }
  ]
}

Extending Schemas

You can extend the schema system with custom validation methods. This is useful for adding domain-specific validations like email or URL formats.

Basic Extension Example

import { extend, type SchemaType, type StringSchemaInterface } from '@esmj/schema';

// First, declare the new methods you want to add
declare module '@esmj/schema' {
  interface StringSchemaInterface {
    email(): StringSchemaInterface;
    url(): StringSchemaInterface;
    trim(): StringSchemaInterface;
  }
}

// Define validation patterns
const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
const URL_REGEX = /^(https?:\/\/[^\s$.?#].[^\s]*)$/;

// Extend the schema system
extend((schema: SchemaType, _, options) => {
  // Only add methods to string schemas
  if (options?.type === 'string') {
    const stringSchema = schema as StringSchemaInterface;
    
    // Add email validation
    stringSchema.email = function() {
      return this.refine((value) => EMAIL_REGEX.test(value), {
        message: 'Invalid email format'
      });
    };
    
    // Add URL validation
    stringSchema.url = function() {
      return this.refine((value) => URL_REGEX.test(value), {
        message: 'Invalid URL format'
      });
    };
    
    // Add string trimming
    stringSchema.trim = function() {
      return this.transform((value) => value.trim());
    };
  }

  return schema;
});

Usage of Extended Schemas

Once extended, you can use your custom methods in schema definitions:

const userSchema = s.object({
  name: s.string().trim(),
  email: s.string().email(),
  website: s.string().url().optional()
});

// Valid data
userSchema.parse({
  name: '  John Doe  ', // Will be trimmed
  email: 'john@example.com'
});

// Invalid data
try {
  userSchema.parse({
    name: 'John Doe',
    email: 'not-an-email'
  });
} catch (error) {
  console.error(error); // "Invalid email format"
}

Advanced Extensions

You can extend any schema type and add complex validations:

declare module '@esmj/schema' {
  interface NumberSchemaInterface {
    positive(): NumberSchemaInterface;
    range(min: number, max: number): NumberSchemaInterface;
  }
  
  interface ArraySchemaInterface<T> {
    minLength(length: number): ArraySchemaInterface<T>;
    unique(): ArraySchemaInterface<T>;
  }
}

extend((schema: SchemaType, _, options) => {
  if (options?.type === 'number') {
    const numberSchema = schema as NumberSchemaInterface;
    
    numberSchema.positive = function() {
      return this.refine((value) => value > 0, {
        message: 'Number must be positive'
      });
    };
    
    numberSchema.range = function(min, max) {
      return this.refine((value) => value >= min && value <= max, {
        message: `Number must be between ${min} and ${max}`
      });
    };
  }
  
  if (options?.type === 'array') {
    const arraySchema = schema as ArraySchemaInterface<unknown>;
    
    arraySchema.minLength = function(length) {
      return this.refine((value) => value.length >= length, {
        message: `Array must contain at least ${length} items`
      });
    };
    
    arraySchema.unique = function() {
      return this.refine((value) => {
        const seen = new Set();
        return value.every(item => {
          const serialized = JSON.stringify(item);
          if (seen.has(serialized)) return false;
          seen.add(serialized);
          return true;
        });
      }, { message: 'Array items must be unique' });
    };
  }
  
  return schema;
});

This extension system gives you the flexibility to create domain-specific validation rules while maintaining type safety and the fluent API style.

More Examples

Nested Objects

You can define schemas for deeply nested objects.

const nestedSchema = s.object({
  user: s.object({
    id: s.number(),
    profile: s.object({
      name: s.string(),
      age: s.number().optional(),
    }),
  }),
});

const result = nestedSchema.parse({
  user: {
    id: 1,
    profile: {
      name: 'John Doe',
    },
  },
});

console.log(result);
// {
//   user: {
//     id: 1,
//     profile: {
//       name: 'John Doe',
//     },
//   },
// }

Arrays with Validation

You can validate arrays with specific item schemas.

const arraySchema = s.array(s.object({ id: s.number(), name: s.string() }));

const result = arraySchema.parse([
  { id: 1, name: 'Item 1' },
  { id: 2, name: 'Item 2' },
]);

console.log(result);
// [
//   { id: 1, name: 'Item 1' },
//   { id: 2, name: 'Item 2' },
// ]

Preprocessing Values

Use s.preprocess to transform input values before validation.

const preprocessSchema = s.preprocess(
  (value) => value.trim(),
  s.string().refine((val) => val.length > 0, { message: 'String cannot be empty' }),
);

const result = preprocessSchema.parse('   hello   ');

console.log(result);
// 'hello'

Transforming Values

Use transform to modify the parsed value.

const transformSchema = s.string().transform((value) => value.toUpperCase());

const result = transformSchema.parse('hello');

console.log(result);
// 'HELLO'

Piping Schemas

Pipe the output of one schema into another for further validation or transformation.

const pipedSchema = s.string()
  .transform((value) => Number.parseInt(value))
  .pipe(s.number().refine((val) => val > 0, { message: 'Number must be positive' }));

const result = pipedSchema.parse('42');

console.log(result);
// 42

Refining Values

Add custom validation logic with refine.

const refinedSchema = s.string().refine((val) => val.startsWith('A'), {
  message: 'String must start with "A"',
});

const result = refinedSchema.parse('Apple');

console.log(result);
// 'Apple'

Default Values

Set default values for optional fields.

const defaultSchema = s.object({
  name: s.string().default('Anonymous'),
  age: s.number().optional().default(18),
});

const result = defaultSchema.parse({});

console.log(result);
// { name: 'Anonymous', age: 18 }

Safe Parsing

Use safeParse to handle errors gracefully.

const safeSchema = s.number();

const result = safeSchema.safeParse('not a number');

if (!result.success) {
  console.error(result.error.message);
} else {
  console.log(result.data);
}
// Error: The value "not a number" must be type of number but is type of "string".

Combining Multiple Features

Combine multiple features like preprocessing, transformations, and refinements.

const combinedSchema = s.preprocess(
  (value) => value.trim(),
  s.string()
    .transform((value) => value.toUpperCase())
    .refine((val) => val.length <= 10, { message: 'String must be at most 10 characters' }),
);

const result = combinedSchema.parse('   hello   ');

console.log(result);
// 'HELLO'

Examples Folder

The examples/ folder contains comprehensive, runnable examples demonstrating various use cases. See the examples README for detailed documentation.

Basic Usage (`examples/basic-usage.ts`)

Demonstrates the core validation features with strings, numbers, arrays, and unions:

node --experimental-strip-types examples/basic-usage.ts

Custom Validation (`examples/custom-validation.ts`)

Shows how to create custom validators for common use cases:

Email validation with regex
URL validation
Age range validation
Password strength validation
Cross-field validation (e.g., password confirmation)

node --experimental-strip-types examples/custom-validation.ts

Advanced Forms (`examples/advanced-forms.ts`)

Real-world form validation examples:

User profile schema with nested objects
Address validation with postal codes
Phone number formatting and validation
API response validation
Complex nested structures

node --experimental-strip-types examples/advanced-forms.ts

Custom Extensions (`examples/custom-extensions.ts`)

Demonstrates how to extend the library with custom methods:

Email validation extension
URL validation extension
UUID validation extension
Combining custom extensions with built-in validators

node --experimental-strip-types examples/custom-extensions.ts

Registration Form (`examples/registration-form.ts`)

Complete user registration form validation with email and phone number validation:

Username validation with pattern matching
Email validation using custom extension
International phone number validation
Password strength requirements
Password confirmation matching
Age verification (18+)
Terms acceptance validation
Error collection with abortEarly: false

node --experimental-strip-types examples/registration-form.ts

To run all examples:

# Using Node.js with experimental type stripping (built-in, no dependencies)
node --experimental-strip-types examples/basic-usage.ts
node --experimental-strip-types examples/custom-validation.ts
node --experimental-strip-types examples/advanced-forms.ts
node --experimental-strip-types examples/custom-extensions.ts
node --experimental-strip-types examples/registration-form.ts

# OR using npm scripts from examples folder
cd examples
npm install
npm run basic
npm run custom
npm run advanced
npm run extensions
npm run registration
npm run all  # Run all examples

# OR using tsx (requires installation)
npm install -g tsx  # If not already installed
npx tsx examples/basic-usage.ts
npx tsx examples/custom-validation.ts
npx tsx examples/advanced-forms.ts
npx tsx examples/custom-extensions.ts
npx tsx examples/registration-form.ts

Migration Guide

From Zod

@esmj/schema has a similar API to Zod, making migration straightforward:

// Zod
import { z } from 'zod';

const userSchema = z.object({
  name: z.string().min(3).max(50),
  email: z.string().email(),
  age: z.number().positive().int(),
  role: z.enum(['admin', 'user']),
  tags: z.array(z.string()).optional()
});

// @esmj/schema (with extensions)
import { s } from '@esmj/schema/full';

const userSchema = s.object({
  name: s.string().min(3).max(50),
  email: s.string(), // Note: email() validation requires custom extension
  age: s.number().positive().int(),
  role: s.enum(['admin', 'user']),
  tags: s.array(s.string()).optional()
});

Key Differences:

Feature	Zod	@esmj/schema
Import	`import { z } from 'zod'`	`import { s } from '@esmj/schema'`
Extensions	Built-in	Modular (`/string`, `/number`, `/array`, `/full`)
Bundle size	~13 KB	~1.4 KB (core), ~4 KB (full)
Email validation	`.email()` built-in	Custom extension (see Extending Schemas)
Error format	Native Error	Plain object `{ success, error, errors }`
Coerce	`z.coerce.number()`	`s.coerce.number()`
Smart cast	No direct equivalent	`s.cast.number()` — rejects nulls, understands `'yes'/'no'`, etc.

Migration Tips:

Replace z with s in your imports
For string methods like .min(), .trim(), import from @esmj/schema/full or @esmj/schema/string
Add custom extensions for email, URL validation (see examples below)
Update error handling to use the plain object structure

From Yup

Migrating from Yup requires a few adjustments in syntax:

// Yup
import * as yup from 'yup';

const userSchema = yup.object({
  name: yup.string().required().min(3).max(50),
  email: yup.string().required().email(),
  age: yup.number().required().positive().integer(),
  website: yup.string().url().nullable(),
  tags: yup.array().of(yup.string()).min(1)
});

// @esmj/schema (with extensions)
import { s } from '@esmj/schema/full';

const userSchema = s.object({
  name: s.string().min(3).max(50), // Fields are required by default
  email: s.string(), // Note: email() validation requires custom extension
  age: s.number().positive().int(),
  website: s.string().nullable(),
  tags: s.array(s.string()).min(1)
});

Key Differences:

Feature	Yup	@esmj/schema
Required fields	`.required()` explicit	Required by default
Optional fields	Default behavior	`.optional()` explicit
Array of type	`.array().of(type)`	`.array(type)`
Integer	`.integer()`	`.int()`
Email validation	`.email()` built-in	Custom extension needed
Async validation	Supported	Not currently supported

Migration Tips:

Remove .required() calls (fields are required by default)
Add .optional() for optional fields
Change .array().of(type) to .array(type)
Change .integer() to .int()
Add custom extensions for email, URL validation

License

MIT

Keywords

FAQs

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Package last updated on 13 Mar 2026

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Install

@esmj/schema

Schema

Table of Contents

Installation

Quick Start

Why Use @esmj/schema?

Performance Highlights

Comparison with Similar Libraries

Performance Comparison

Schema Creation Performance

Parsing Performance

Error Handling Performance

Usage

Basic Usage

Modular Extensions

Import Options

Bundle Size Impact

String Extensions (@esmj/schema/string)

Number Extensions (@esmj/schema/number)

Array Extensions (@esmj/schema/array)

Full Extensions (@esmj/schema/full)

API Reference Summary

Core Types

Modifiers

Coerce

Cast

Transformations

String Extensions

Number Extensions

Array Extensions

Schema Types

s.string(options?)

s.number(options?)

s.boolean(options?)

s.date(options?)

s.object(definition, options?)

s.array(definition, options?)

s.enum(values, options?)

s.literal(value, options?)

s.union(definitions, options?)

s.any()

s.preprocess(callback, schema)

s.coerce

s.cast